linux-kernel - Re: [PATCH] iio: amplifiers: ada4250: use DMA-safe memory for regmap_bulk

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Message-ID: <3875da4e070f991484b8536f6066347615b0815a.camel@gmail.com>
Date: Mon, 21 Apr 2025 14:00:30 +0100
From: Nuno Sá <noname.nuno@...il.com>
To: Jonathan Cameron <jic23@...nel.org>, David Lechner
 <dlechner@...libre.com>
Cc: Andy Shevchenko <andy.shevchenko@...il.com>, Antoniu Miclaus	
 <antoniu.miclaus@...log.com>, Lars-Peter Clausen <lars@...afoo.de>, Michael
 Hennerich <Michael.Hennerich@...log.com>, Nuno Sá
 <nuno.sa@...log.com>,  Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>, 
 "linux-kernel@...r.kernel.org"	 <linux-kernel@...r.kernel.org>, Mark Brown
 <broonie@...nel.org>
Subject: Re: [PATCH] iio: amplifiers: ada4250: use DMA-safe memory for
 regmap_bulk_read()

On Mon, 2025-04-21 at 11:57 +0100, Jonathan Cameron wrote:
> On Sat, 19 Apr 2025 13:07:25 -0500
> David Lechner <dlechner@...libre.com> wrote:
> 
> > On 4/19/25 10:03 AM, Andy Shevchenko wrote:
> > > On Fri, Apr 18, 2025 at 02:09:04PM -0500, David Lechner wrote:  
> > > > On 4/18/25 1:58 PM, Andy Shevchenko wrote:  
> > > > > пʼятниця, 18 квітня 2025 р. David Lechner
> > > > > <dlechner@...libre.com <mailto:dlechner@...libre.com>> пише:
> > > > > 
> > > > >     Use DMA-safe memory instead of stack-allocated memory for the call
> > > > > to
> > > > >     regmap_bulk_read() in the ada4250_init() function as this could be
> > > > > used
> > > > >     directly by a SPI controller.
> > > > > 
> > > > > Sorry, but can you elaborate more on this? If driver doesn’t override
> > > > > the
> > > > > callbacks the regmap SPI uses spi_write_then_read() which is supposed
> > > > > to be
> > > > > dma safe.   
> > > > 
> > > > Ah, I didn't dig that far down. Will send a new patch that just cleans
> > > > up the
> > > > unnecessary alignment and unaligned call.  
> > > 
> > > But do you have a real life issue with that? Coincidentally we discussed
> > > similar case in another driver with colleague of mine, and he insists that
> > > it
> > > might be still broken (somewhere).
> > >   
> > 
> > Some drivers that use regmap with SPI implement their own read and write
> > callbacks if the default one doesn't work, so maybe something like that?
> > 
> 
> I asked Mark about this long ago. Maybe the guidance has changed, but the
> request
> if my memory is correct was to treat bulk regmap accesses with the same care
> wrt to
> DMA safety that we treat direct use of the underlying bus accessors.
> (unfortunately I have no idea what thread this was in!)
> 
> The reasoning I think was that it was an implementation detail whether
> regmap copied the data or not for bulk accesses.  Back then I chased through
> the SPI case and couldn't find a path that didn't bounce the data (though some
> paths could have been made zero copy).
> 

Looking on the implementation, indeed it looks like that everything should be
fine... However, IMO, David's reasoning about potential drivers using their own
regmap_bus (or just overriding read and write) is perhaps justification enough
for doing it.

- Nuno Sá 

> Anyhow, +CC Mark as maybe I remembered that wrong or his view has
> changed!
> 
> Jonathan