[alsa-devel] Incorrect buffer handling in dw-hdmi bridge audio

[Russell King - ARM Linux admin]

On Tue, Nov 05, 2019 at 05:44:26PM +0100, Takashi Iwai wrote:
> On Tue, 05 Nov 2019 17:33:44 +0100,
> Takashi Iwai wrote:
> > 
> > On Tue, 05 Nov 2019 17:02:15 +0100,
> > Russell King - ARM Linux admin wrote:
> > > 
> > > On Tue, Nov 05, 2019 at 09:07:43AM +0100, Neil Armstrong wrote:
> > > > Hi,
> > > > 
> > > > On 05/11/2019 08:55, Takashi Iwai wrote:
> > > > > Hi,
> > > > > 
> > > > > while recently working on the ALSA memory allocator API cleanup, I
> > > > > noticed that dw-hdmi bridge driver seems doing weird about the buffer
> > > > > management.  It pre-allocates the usual device buffers fully at the
> > > > > probe time, while each stream allocates the buffer via the vmalloc
> > > > > helpers and replaces with it at each open.
> > > > > 
> > > > > I guess it's no expected behavior?  It's basically a full waste of
> > > > > resources, and the vmalloc buffer isn't guaranteed to work well for
> > > > > mmap on every architecture.
> > > > > 
> > > > > Below is the patch to address it.  Can anyone check whether this still
> > > > > works?
> > > > 
> > > > I don't have the setup to check, but this has been pushed by Russell I Added in CC.
> > > > 
> > > > I also added the imx maintainer since it's (AFAIK) only used on iMX SoCs.
> > > > 
> > > > Neil
> > > > 
> > > > > 
> > > > > Since I have a cleanup series and this is involved, I'd like to take
> > > > > the fix through my tree once after it's confirmed (and get ACK if
> > > > > possible).
> > > > > 
> > > > > 
> > > > > Thanks!
> > > > > 
> > > > > Takashi
> > > > > 
> > > > > -- 8< --
> > > > > From: Takashi Iwai <tiwai at suse.de>
> > > > > Subject: [PATCH] drm/bridge: dw-hdmi: Fix the incorrect buffer allocations
> > > > > 
> > > > > The driver sets up the buffer preallocation with SNDRV_DMA_TYPE_DEV,
> > > > > while it re-allocates and releases vmalloc pages.  This is not only a
> > > > > lot of waste resources but also causes the mmap malfunction.
> > > > > 
> > > > > Change / drop the vmalloc-related code and use the standard buffer
> > > > > allocation / release code instead.
> > > 
> > > I think getting rid of the vmalloc code here is a mistake - I seem to
> > > remember using the standard buffer allocation causes failures, due to
> > > memory fragmentation.  Since the hardware is limited to DMA from at
> > > most one page, there is no reason for this driver to require contiguous
> > > pages, hence why it's using - and should use - vmalloc pages.  vmalloc
> > > is way kinder to the MM subsystem than trying to request large order
> > > contiguous pages.
> > > 
> > > So, NAK on this patch.
> > 
> > OK, then we should do other way round, rather drop the buffer
> > preallocation instead.  Currently vmalloc buffer is always allocated
> > at each open and overrides the preallocated buffer, so the whole 64k
> > and more are wasted for no use.
> > 
> > (BTW, the current code has this snippet:
> > 
> > 	/* Limit the buffer size to the size of the preallocated buffer */
> > 	ret = snd_pcm_hw_constraint_minmax(runtime,
> > 					   SNDRV_PCM_HW_PARAM_BUFFER_SIZE,
> > 					   0, substream->dma_buffer.bytes);
> > 
> > ... and this would have to limit the buffer size only to the
> > preallocated size -- which essentially makes the argument above
> > invalid.  However, this check looks actually bogus, the constraint
> > parameter should be SNDRV_PCM_HW_PARAM_BUFFER_BYTES, not _SIZE.  It
> > might be the reason it worked somehow...)
> > 
> > So below is the revised patch.  Could you guys check it again?
> > 
> > There I copied the comment as is, although the 512k mentioned there
> > looks inconsistent with the actual code.  Should it be 1M?
> 
> ... and reading the patch again, I found that the hw constraint call
> can be dropped as well.  The dw_hdmi_hw definition already contains
> the max buffer size.
> 
> Below is the re-revised patch.  Please check it.

I was slightly wrong - sorry.  It's been a long time since I looked at
this driver, or even used it - but it is the only driver that supports
HDMI audio on iMX6 platforms, so I guess there are lots of users out
there... or maybe not if none of them use mainline kernels.

The hardware is capable of reading across a page.  However, the hardware
is _not_ capable of reading any data that is formatted as ALSA APIs
allow it to be.  The driver has to reformat the ALSA supplied sound
buffers to the layout the hardware requires.

To do this, we have two different buffers:

- The substream buffer is the buffer which the hardware reads from.
- The runtime buffer is the buffer which ALSA uses.

The call to snd_pcm_lib_preallocate_pages_for_all() allocates the
hardware buffer, which is a single contiguous buffer of fixed size.

The user buffer is allocated with snd_pcm_lib_alloc_vmalloc_buffer().

Hence, the driver makes use of both.  You can't get rid of either
of them.

-- 
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