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

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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.

-- 
With Best Regards,
Andy Shevchenko


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