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

From: Andy Shevchenko <andriy.shevchenko@intel.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>,
	David Lechner <david@lechnology.com>,
	dri-devel <dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH v2 05/11] drm/tinydrm: Remove tinydrm_spi_max_transfer_size()
Date: Wed, 16 Oct 2019 11:07:54 +0300	[thread overview]
Message-ID: <20191016080754.GS32742@smile.fi.intel.com> (raw)
In-Reply-To: <CAKMK7uEW83M0G8TuSMsVysGLc6eK27sGgUvYq=kr8mBgNuAoSg@mail.gmail.com>

On Tue, Oct 15, 2019 at 06:05:16PM +0200, Daniel Vetter wrote:
> On Tue, Oct 15, 2019 at 5:59 PM Andy Shevchenko
> <andriy.shevchenko@intel.com> wrote:
> > On Tue, Oct 15, 2019 at 05:41:53PM +0200, Noralf Trønnes wrote:
> > > Den 15.10.2019 16.32, skrev Andy Shevchenko:
> > > > 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.
> > > >
> > >
> > > Isn't this a spi controller driver problem?
> >
> > It doesn't matter. This patch made a regression. Before it worked,
> > now it doesn't.
> 
> Yes this is clear.
> 
> > > spi_max_transfer_size() tells the client what the maximum transfer
> > > length is. The controller driver can use ctlr->max_transfer_size if it
> > > has restrictions.
> >
> > It might be a problem of the SPI core.
> >
> > > AFAIUI max_dma_len is used when splitting up the buffer for the sg table
> > > in spi_map_buf().
> >
> > > > Please, fix.
> >
> > Should I send the revert?
> 
> Can we please not roll in with the cavalry before everyone had a
> chance to wake up and look at this properly? It's less than 1h since
> your initial bug report.

It was a simple question to the 1st level maintainer of the subsystem,
who already read the report and answered to it.

> Also, you _do_ have a test patch in your inbox ...

Yes, thanks for it, I'm going to test right now.

-- 
With Best Regards,
Andy Shevchenko

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