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

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Message-ID: <20250421115758.62c16f94@jic23-huawei>
Date: Mon, 21 Apr 2025 11:57:58 +0100
From: Jonathan Cameron <jic23@...nel.org>
To: 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 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).

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

Jonathan