[PATCH v2 0/2] Chunk splitting of spi transfers

[Noralf Trønnes]

Den 02.03.2018 12.11, skrev Meghana Madhyastha:
> On Sun, Feb 25, 2018 at 02:19:10PM +0100, Lukas Wunner wrote:
>> [cc += linux-rpi-kernel at lists.infradead.org]
>>
>> On Sat, Feb 24, 2018 at 06:15:59PM +0000, Meghana Madhyastha wrote:
>>> I've added bcm2835_spi_transfer_one_message in spi-bcm2835. This calls
>>> spi_split_transfers_maxsize to split large chunks for spi dma transfers.
>>> I then removed chunk splitting in the tinydrm spi helper (as now the core
>>> is handling the chunk splitting). However, although the SPI HW should be
>>> able to accomodate up to 65535 bytes for dma transfers, the splitting of
>>> chunks to 65535 bytes results in a dma transfer time out error. However,
>>> when the chunks are split to < 64 bytes it seems to work fine.
>> Hm, that is really odd, how did you test this exactly, what did you
>> use as SPI slave?  It contradicts our own experience, we're using
>> Micrel KSZ8851 Ethernet chips as SPI slave on spi0 of a BCM2837
>> and can send/receive messages via DMA to the tune of several hundred
>> bytes without any issues.  In fact, for messages < 96 bytes, DMA is
>> not used at all, so you've probably been using interrupt mode,
>> see the BCM2835_SPI_DMA_MIN_LENGTH macro in spi-bcm2835.c.
> Hi Lukas,
>
> I think you are right. I checked it and its not using the DMA mode which
> is why its working with 64 bytes.
> Noralf, that leaves us back to the
> initial time out problem. I've tried doing the message splitting in
> spi_sync as well as spi_pump_messages. Martin had explained that DMA
> will wait for
> the SPI HW to set the send_more_data line, but the SPI-HW itself will
> stop triggering it when SPI_LEN is 0 causing DMA to wait forever. I
> thought if we split it before itself, the SPI_LEN will not go to zero
> thus preventing this problem, however it didn't work and started
> hanging. So I'm a little uncertain as to how to proceed and debug what
> exactly has caused the time out due to the asynchronous methods.

I did a quick test and at least this is working:

int tinydrm_spi_transfer(struct spi_device *spi, u32 speed_hz,
              struct spi_transfer *header, u8 bpw, const void *buf,
              size_t len)
{
     struct spi_transfer tr = {
         .bits_per_word = bpw,
         .speed_hz = speed_hz,
         .tx_buf = buf,
         .len = len,
     };
     struct spi_message m;
     size_t maxsize;
     int ret;

     maxsize = tinydrm_spi_max_transfer_size(spi, 0);

     if (drm_debug & DRM_UT_DRIVER)
         pr_debug("[drm:%s] bpw=%u, maxsize=%zu, transfers:\n",
              __func__, bpw, maxsize);

     spi_message_init(&m);
     m.spi = spi;
     if (header)
         spi_message_add_tail(header, &m);
     spi_message_add_tail(&tr, &m);

     ret = spi_split_transfers_maxsize(spi->controller, &m, maxsize, 
GFP_KERNEL);
     if (ret)
         return ret;

     tinydrm_dbg_spi_message(spi, &m);

     return spi_sync(spi, &m);
}
EXPORT_SYMBOL(tinydrm_spi_transfer);


Log:
[   39.015644] [drm:mipi_dbi_fb_dirty [mipi_dbi]] Flushing [FB:36] x1=0, 
x2=320, y1=0, y2=240

[   39.018079] [drm:mipi_dbi_typec3_command [mipi_dbi]] cmd=2a, par=00 
00 01 3f
[   39.018129] [drm:tinydrm_spi_transfer] bpw=8, maxsize=65532, transfers:
[   39.018152]     tr(0): speed=10MHz, bpw=8, len=1, tx_buf=[2a]
[   39.018231] [drm:tinydrm_spi_transfer] bpw=8, maxsize=65532, transfers:
[   39.018248]     tr(0): speed=10MHz, bpw=8, len=4, tx_buf=[00 00 01 3f]

[   39.018330] [drm:mipi_dbi_typec3_command [mipi_dbi]] cmd=2b, par=00 
00 00 ef
[   39.018347] [drm:tinydrm_spi_transfer] bpw=8, maxsize=65532, transfers:
[   39.018362]     tr(0): speed=10MHz, bpw=8, len=1, tx_buf=[2b]
[   39.018396] [drm:tinydrm_spi_transfer] bpw=8, maxsize=65532, transfers:
[   39.018428]     tr(0): speed=10MHz, bpw=8, len=4, tx_buf=[00 00 00 ef]

[   39.018487] [drm:mipi_dbi_typec3_command [mipi_dbi]] cmd=2c, len=153600
[   39.018502] [drm:tinydrm_spi_transfer] bpw=8, maxsize=65532, transfers:
[   39.018517]     tr(0): speed=10MHz, bpw=8, len=1, tx_buf=[2c]
[   39.018565] [drm:tinydrm_spi_transfer] bpw=8, maxsize=65532, transfers:
[   39.018594]     tr(0): speed=48MHz, bpw=8, len=65532, tx_buf=[c6 18 
c6 18 c6 18 c6 18 c6 18 c6 18 c6 18 c6 18 ...]
[   39.018608]     tr(1): speed=48MHz, bpw=8, len=65532, tx_buf=[06 18 
06 18 06 18 06 18 06 18 06 18 06 18 06 18 ...]
[   39.018621]     tr(2): speed=48MHz, bpw=8, len=22536, tx_buf=[10 82 
10 82 10 82 10 82 10 82 10 82 18 e3 18 e3 ...]


Noralf.