| The Spidernet Device Driver
|Written by Linas Vepstas <firstname.lastname@example.org>
|Version of 7 June 2007
|This document sketches the structure of portions of the spidernet
|device driver in the Linux kernel tree. The spidernet is a gigabit
|ethernet device built into the Toshiba southbridge commonly used
|in the SONY Playstation 3 and the IBM QS20 Cell blade.
|The Structure of the RX Ring.
|The receive (RX) ring is a circular linked list of RX descriptors,
|together with three pointers into the ring that are used to manage its
|The elements of the ring are called "descriptors" or "descrs"; they
|describe the received data. This includes a pointer to a buffer
|containing the received data, the buffer size, and various status bits.
|There are three primary states that a descriptor can be in: "empty",
|"full" and "not-in-use". An "empty" or "ready" descriptor is ready
|to receive data from the hardware. A "full" descriptor has data in it,
|and is waiting to be emptied and processed by the OS. A "not-in-use"
|descriptor is neither empty or full; it is simply not ready. It may
|not even have a data buffer in it, or is otherwise unusable.
|During normal operation, on device startup, the OS (specifically, the
|spidernet device driver) allocates a set of RX descriptors and RX
|buffers. These are all marked "empty", ready to receive data. This
|ring is handed off to the hardware, which sequentially fills in the
|buffers, and marks them "full". The OS follows up, taking the full
|buffers, processing them, and re-marking them empty.
|This filling and emptying is managed by three pointers, the "head"
|and "tail" pointers, managed by the OS, and a hardware current
|descriptor pointer (GDACTDPA). The GDACTDPA points at the descr
|currently being filled. When this descr is filled, the hardware
|marks it full, and advances the GDACTDPA by one. Thus, when there is
|flowing RX traffic, every descr behind it should be marked "full",
|and everything in front of it should be "empty". If the hardware
|discovers that the current descr is not empty, it will signal an
|interrupt, and halt processing.
|The tail pointer tails or trails the hardware pointer. When the
|hardware is ahead, the tail pointer will be pointing at a "full"
|descr. The OS will process this descr, and then mark it "not-in-use",
|and advance the tail pointer. Thus, when there is flowing RX traffic,
|all of the descrs in front of the tail pointer should be "full", and
|all of those behind it should be "not-in-use". When RX traffic is not
|flowing, then the tail pointer can catch up to the hardware pointer.
|The OS will then note that the current tail is "empty", and halt
|The head pointer (somewhat mis-named) follows after the tail pointer.
|When traffic is flowing, then the head pointer will be pointing at
|a "not-in-use" descr. The OS will perform various housekeeping duties
|on this descr. This includes allocating a new data buffer and
|dma-mapping it so as to make it visible to the hardware. The OS will
|then mark the descr as "empty", ready to receive data. Thus, when there
|is flowing RX traffic, everything in front of the head pointer should
|be "not-in-use", and everything behind it should be "empty". If no
|RX traffic is flowing, then the head pointer can catch up to the tail
|pointer, at which point the OS will notice that the head descr is
|"empty", and it will halt processing.
|Thus, in an idle system, the GDACTDPA, tail and head pointers will
|all be pointing at the same descr, which should be "empty". All of the
|other descrs in the ring should be "empty" as well.
|The show_rx_chain() routine will print out the the locations of the
|GDACTDPA, tail and head pointers. It will also summarize the contents
|of the ring, starting at the tail pointer, and listing the status
|of the descrs that follow.
|A typical example of the output, for a nearly idle system, might be
|net eth1: Total number of descrs=256
|net eth1: Chain tail located at descr=20
|net eth1: Chain head is at 20
|net eth1: HW curr desc (GDACTDPA) is at 21
|net eth1: Have 1 descrs with stat=x40800101
|net eth1: HW next desc (GDACNEXTDA) is at 22
|net eth1: Last 255 descrs with stat=xa0800000
|In the above, the hardware has filled in one descr, number 20. Both
|head and tail are pointing at 20, because it has not yet been emptied.
|Meanwhile, hw is pointing at 21, which is free.
|The "Have nnn decrs" refers to the descr starting at the tail: in this
|case, nnn=1 descr, starting at descr 20. The "Last nnn descrs" refers
|to all of the rest of the descrs, from the last status change. The "nnn"
|is a count of how many descrs have exactly the same status.
|The status x4... corresponds to "full" and status xa... corresponds
|to "empty". The actual value printed is RXCOMST_A.
|In the device driver source code, a different set of names are
|used for these same concepts, so that
|"empty" == SPIDER_NET_DESCR_CARDOWNED == 0xa
|"full" == SPIDER_NET_DESCR_FRAME_END == 0x4
|"not in use" == SPIDER_NET_DESCR_NOT_IN_USE == 0xf
|The RX RAM full bug/feature
|As long as the OS can empty out the RX buffers at a rate faster than
|the hardware can fill them, there is no problem. If, for some reason,
|the OS fails to empty the RX ring fast enough, the hardware GDACTDPA
|pointer will catch up to the head, notice the not-empty condition,
|ad stop. However, RX packets may still continue arriving on the wire.
|The spidernet chip can save some limited number of these in local RAM.
|When this local ram fills up, the spider chip will issue an interrupt
|indicating this (GHIINT0STS will show ERRINT, and the GRMFLLINT bit
|will be set in GHIINT1STS). When the RX ram full condition occurs,
|a certain bug/feature is triggered that has to be specially handled.
|This section describes the special handling for this condition.
|When the OS finally has a chance to run, it will empty out the RX ring.
|In particular, it will clear the descriptor on which the hardware had
|stopped. However, once the hardware has decided that a certain
|descriptor is invalid, it will not restart at that descriptor; instead
|it will restart at the next descr. This potentially will lead to a
|deadlock condition, as the tail pointer will be pointing at this descr,
|which, from the OS point of view, is empty; the OS will be waiting for
|this descr to be filled. However, the hardware has skipped this descr,
|and is filling the next descrs. Since the OS doesn't see this, there
|is a potential deadlock, with the OS waiting for one descr to fill,
|while the hardware is waiting for a different set of descrs to become
|A call to show_rx_chain() at this point indicates the nature of the
|problem. A typical print when the network is hung shows the following:
|net eth1: Spider RX RAM full, incoming packets might be discarded!
|net eth1: Total number of descrs=256
|net eth1: Chain tail located at descr=255
|net eth1: Chain head is at 255
|net eth1: HW curr desc (GDACTDPA) is at 0
|net eth1: Have 1 descrs with stat=xa0800000
|net eth1: HW next desc (GDACNEXTDA) is at 1
|net eth1: Have 127 descrs with stat=x40800101
|net eth1: Have 1 descrs with stat=x40800001
|net eth1: Have 126 descrs with stat=x40800101
|net eth1: Last 1 descrs with stat=xa0800000
|Both the tail and head pointers are pointing at descr 255, which is
|marked xa... which is "empty". Thus, from the OS point of view, there
|is nothing to be done. In particular, there is the implicit assumption
|that everything in front of the "empty" descr must surely also be empty,
|as explained in the last section. The OS is waiting for descr 255 to
|become non-empty, which, in this case, will never happen.
|The HW pointer is at descr 0. This descr is marked 0x4.. or "full".
|Since its already full, the hardware can do nothing more, and thus has
|halted processing. Notice that descrs 0 through 254 are all marked
|"full", while descr 254 and 255 are empty. (The "Last 1 descrs" is
|descr 254, since tail was at 255.) Thus, the system is deadlocked,
|and there can be no forward progress; the OS thinks there's nothing
|to do, and the hardware has nowhere to put incoming data.
|This bug/feature is worked around with the spider_net_resync_head_ptr()
|routine. When the driver receives RX interrupts, but an examination
|of the RX chain seems to show it is empty, then it is probable that
|the hardware has skipped a descr or two (sometimes dozens under heavy
|network conditions). The spider_net_resync_head_ptr() subroutine will
|search the ring for the next full descr, and the driver will resume
|operations there. Since this will leave "holes" in the ring, there
|is also a spider_net_resync_tail_ptr() that will skip over such holes.
|As of this writing, the spider_net_resync() strategy seems to work very
|well, even under heavy network loads.
|The TX ring
|The TX ring uses a low-watermark interrupt scheme to make sure that
|the TX queue is appropriately serviced for large packet sizes.
|For packet sizes greater than about 1KBytes, the kernel can fill
|the TX ring quicker than the device can drain it. Once the ring
|is full, the netdev is stopped. When there is room in the ring,
|the netdev needs to be reawakened, so that more TX packets are placed
|in the ring. The hardware can empty the ring about four times per jiffy,
|so its not appropriate to wait for the poll routine to refill, since
|the poll routine runs only once per jiffy. The low-watermark mechanism
|marks a descr about 1/4th of the way from the bottom of the queue, so
|that an interrupt is generated when the descr is processed. This
|interrupt wakes up the netdev, which can then refill the queue.
|For large packets, this mechanism generates a relatively small number
|of interrupts, about 1K/sec. For smaller packets, this will drop to zero
|interrupts, as the hardware can empty the queue faster than the kernel
|can fill it.
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