Re: [PATCH v2 05/11] drm/tinydrm: Remove tinydrm_spi_max_transfer_size()

From: Andy Shevchenko <andriy.shevchenko@intel.com>
To: "Noralf Trønnes" <noralf@tronnes.org>
Cc: sam@ravnborg.org, david@lechnology.com, dri-devel@lists.freedesktop.org
Subject: Re: [PATCH v2 05/11] drm/tinydrm: Remove tinydrm_spi_max_transfer_size()
Date: Tue, 15 Oct 2019 18:02:57 +0300	[thread overview]
Message-ID: <20191015150257.GK32742@smile.fi.intel.com> (raw)
In-Reply-To: <20191015143236.GA5363@smile.fi.intel.com>

On Tue, Oct 15, 2019 at 05:32:36PM +0300, Andy Shevchenko wrote:
> On Fri, Jul 19, 2019 at 05:59:10PM +0200, Noralf Trønnes wrote:
> > spi-bcm2835 can handle >64kB buffers now so there is no need to check
> > ->max_dma_len. The tinydrm_spi_max_transfer_size() max_len argument is
> > not used by any callers, so not needed.
> > 
> > Then we have the spi_max module parameter. It was added because
> > staging/fbtft has support for it and there was a report that someone used
> > it to set a small buffer size to avoid popping on a USB soundcard on a
> > Raspberry Pi. In hindsight it shouldn't have been added, I should have
> > waited for it to become a problem first. I don't know it anyone is
> > actually using it, but since tinydrm_spi_transfer() is being moved to
> > mipi-dbi, I'm taking the opportunity to remove it. I'll add it back to
> > mipi-dbi if someone complains.
> > 
> > With that out of the way, spi_max_transfer_size() can be used instead.
> > 
> > The chosen 16kB buffer size for Type C Option 1 (9-bit) interface is
> > somewhat arbitrary, but a bigger buffer will have a miniscule impact on
> > transfer speed, so it's probably fine.
> 
> This breaks the SPI PXA2xx case I'm using. The world is not a Pi:e.
> 
> [  388.445752] mi0283qt spi-PRP0001:01: DMA disabled for transfer length 153600 greater than 65536
> [  388.634437] mi0283qt spi-PRP0001:01: DMA disabled for transfer length 153600 greater than 65536
> [  388.822933] mi0283qt spi-PRP0001:01: DMA disabled for transfer length 153600 greater than 65536
> 
> The crucial thing is to check the transfer size against maximum DMA length
> of the master.
> 
> Please, fix.

Partial revert fixes the issue, though I'm not sure it's the best approach.

--- a/drivers/gpu/drm/drm_mipi_dbi.c
+++ b/drivers/gpu/drm/drm_mipi_dbi.c
@@ -1147,7 +1147,7 @@ EXPORT_SYMBOL(mipi_dbi_spi_init);
 int mipi_dbi_spi_transfer(struct spi_device *spi, u32 speed_hz,
 			  u8 bpw, const void *buf, size_t len)
{
-	size_t max_chunk = spi_max_transfer_size(spi);
+	size_t max_chunk = min(spi_max_transfer_size(spi), spi->master->max_dma_len);
 	struct spi_transfer tr = {
 		.bits_per_word = bpw,
 		.speed_hz = speed_hz,

-- 
With Best Regards,
Andy Shevchenko


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