Discussion:
[PATCH v2 1/2] drm/tinydrm: Remove chunk splitting in tinydrm_spi_transfer
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Noralf Trønnes
2018-02-27 17:39:41 UTC
Permalink
-Remove chunk splitting in tinydrm_spi_transfer in tinydrm-helpers as The spi core will split a buffer into max_dma_len chunks for the
spi controller driver to handle.
-Remove automatic byte swapping in tinydrm_spi_transfer as it doesn't have users.
-Remove the upper bound check on dma transfer length in bcm2835_spi_can_dma().
AFAICS you need to reverse the order of the two patches, so that the
splitting is added to spi-bcm2835 before you remove it from the DRM
drivers. Otherwise there's no splitting at all in-between the patches,
which may cause issues for someone hitting this patch when bisecting.
It would be good if you could explain the rationale of the patch
(the "why") in more detail. You've provided the rationale in the
cover letter but the cover letter isn't recorded in the git history.
Right now the commit message only summarizes the "what".
Yeah, the why is important.

Currently clients using spi-bcm2835 need to know about the max_dma_len
limit to ensure DMA transfers. Trying to transfer larger buffers will
result in a warning and a slow PIO transfer.
tinydrm works around this by splitting the framebuffers before transfer
(a legacy from fbtft which tinydrm grew out of).

So the change is about removing the upper bound on DMA SPI transfers for
this particular spi controller driver.

(When I checked this a year ago, there was one more controller driver with
an upper bound on DMA transfers, but I don't remember which)

Noralf.
Also, please wrap the commit message at 72 chars.
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -365,19 +365,6 @@ static bool bcm2835_spi_can_dma(struct spi_master *master,
if (tfr->len < BCM2835_SPI_DMA_MIN_LENGTH)
return false;
- /* BCM2835_SPI_DLEN has defined a max transfer size as
- * 16 bit, so max is 65535
- * we can revisit this by using an alternative transfer
- * method - ideally this would get done without any more
- * interaction...
- */
- if (tfr->len > 65535) {
- dev_warn_once(&spi->dev,
- "transfer size of %d too big for dma-transfer\n",
- tfr->len);
- return false;
- }
-
/* if we run rx/tx_buf with word aligned addresses then we are OK */
if ((((size_t)tfr->rx_buf & 3) == 0) &&
(((size_t)tfr->tx_buf & 3) == 0))
Move this hunk to the other patch so that it's together with the
changes that remove the 65535 limitation. It would probably good
to retain a portion of the comment to explain why the splitting is
necessary.
@@ -461,7 +448,7 @@ static void bcm2835_dma_init(struct spi_master *master, struct device *dev)
/* all went well, so set can_dma */
master->can_dma = bcm2835_spi_can_dma;
- master->max_dma_len = 65535; /* limitation by BCM2835_SPI_DLEN */
+ master->max_dma_len = 65528; /* limitation by BCM2835_SPI_DLEN */
Spurious unexplained change.
Thanks,
Lukas
Noralf Trønnes
2018-02-27 17:40:53 UTC
Permalink
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.
Meghana,
I suggest using modetest with page flipping to check that you don't
regress performance wise:

$ ./libdrm/tests/modetest/modetest -M mi0283qt -s 28:***@RG16 -v
setting mode 320x240-***@RG16 on connectors 28, crtc 30
freq: 29.88Hz
freq: 29.96Hz
freq: 29.99Hz
^C

Noralf.

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