linux-kernel - Re: [PATCH 02/10] regmap: spi: Check raw_[read|write] against max message size

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Date:   Wed, 8 Sep 2021 13:17:45 +0000
From:   Charles Keepax <ckeepax@...nsource.cirrus.com>
To:     Lucas Tanure <tanureal@...nsource.cirrus.com>
CC:     Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Sanjay R Mehta <sanju.mehta@....com>,
        Nehal Bakulchandra Shah <Nehal-Bakulchandra.shah@....com>,
        <linux-kernel@...r.kernel.org>, <linux-spi@...r.kernel.org>,
        <patches@...nsource.cirrus.com>
Subject: Re: [PATCH 02/10] regmap: spi: Check raw_[read|write] against max
 message size

On Wed, Sep 08, 2021 at 01:09:29PM +0000, Charles Keepax wrote:
> On Wed, Sep 08, 2021 at 12:34:43PM +0100, Lucas Tanure wrote:
> > regmap-spi will split data and address between two transfers
> > in the same message, so max_[read|write] must include space
> > for the address and padding
> > 
> > Signed-off-by: Lucas Tanure <tanureal@...nsource.cirrus.com>
> > ---
> >  drivers/base/regmap/regmap-spi.c |  4 ++++
> >  drivers/base/regmap/regmap.c     | 15 +++++++++++++++
> >  include/linux/regmap.h           |  3 +++
> >  3 files changed, 22 insertions(+)
> > 
> > diff --git a/drivers/base/regmap/regmap-spi.c b/drivers/base/regmap/regmap-spi.c
> > index 0e6552e57ecf..1434c502e340 100644
> > --- a/drivers/base/regmap/regmap-spi.c
> > +++ b/drivers/base/regmap/regmap-spi.c
> > @@ -123,6 +123,10 @@ static const struct regmap_bus *regmap_get_spi_bus(struct spi_device *spi,
> >  		bus->free_on_exit = true;
> >  		bus->max_raw_read = max_size;
> >  		bus->max_raw_write = max_size;
> > +
> > +		if (spi_max_message_size(spi) != SIZE_MAX)
> > +			bus->max_combined_rw = spi_max_message_size(spi);
> 
> I am not sure max_combined_rw is the best name here, it makes
> sense in a SPI context where reads are a write followed by a
> read. But does it really make sense for all buses? Like an MMIO
> this no longer seems a very meaningful name.
> 
> Perhaps max_transaction? But I am often not the best at thinking
> of names myself.
> 

Although thinking about this more are we sure this wouldn't just
be better as a flag to include the address in the max_raw_read/write?
I am not sure what extra use-cases the extra max_combined_rw
opens up and it feels like the field is doing two things, 1)
saying that the address needs to be included in the max size and
2) specifying a new max size.

Thanks,
Charles