linux-kernel - Re: [PATCH 1/1] regmap: spi-avmm: Fix regmap_bus max_raw

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Message-ID: <ZJpZJhostb4G1UMt@yilunxu-OptiPlex-7050>
Date:   Tue, 27 Jun 2023 11:36:06 +0800
From:   Xu Yilun <yilun.xu@...el.com>
To:     Jim Wylder <jwylder@...gle.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Russ Weight <russell.h.weight@...el.com>,
        gregkh@...uxfoundation.org, rafael@...nel.org,
        linux-kernel@...r.kernel.org, matthew.gerlach@...ux.intel.com
Subject: Re: [PATCH 1/1] regmap: spi-avmm: Fix regmap_bus max_raw_write

On 2023-06-26 at 15:23:45 -0500, Jim Wylder wrote:
> On Mon, Jun 26, 2023 at 2:47 PM Mark Brown <broonie@...nel.org> wrote:
> >
> > On Sun, Jun 25, 2023 at 12:26:31PM +0800, Xu Yilun wrote:
> >
> > > IIUC, max_raw_write/read is the max batch *DATA* size that could be
> > > handled by the receiver. reg addr bytes are not counted in. I'm not 100%
> > > sure this is obeyed by all drivers. But see several examples:
> >
> > There's clearly been some confusion in a bunch of drivers, those you've
> > identified below need fixing too for the new code from the looks of it.
> > I'm frankly unclear why some of the drivers you're pointing at are even
> > implementing raw buses.
> >
> > > So I'm not sure if commit 3981514180c9 is actually necessary.
> >
> > That's "regmap: Account for register length when chunking".  It's

Yes, will try to be as readable next time.

> > certainly a bit unclear now I go do another survey, though it's also
> > clear that things like the handling of padding are intermittent at best.

Handling of padding is good.

> > We probably would be safe reverting that.
> >
> > Jim, where were you seeing the issue here?
> 
> Hope I am answering your question.
> 
> The issue I experienced is that if a bus (in my case a limited i2c controller)
> defines a quirk with max_raw_write, then the chunking algorithm would
> divide the data into max_raw_write chunks.  The i2c bus would then
> prepend the address values to the chunk which would
> always get rejected because it was at least one byte too large.
> 
> My original fix, that I posted was to add a special flag (reg_in_write)
> that a bus could set to choose the to have the register accounted
> for in the chunking algorithm.  This was admittedly inelegant.
> 
> After reviews, we thought using the reg_bytes would be a better
> solution and that padding should be accounted for.
> 
> I had not seen an issue with padding for this algorithm.  Only
> the case specified above with i2c with prepending the address.
> 
> Would it be possible to reconsider adding a flag or argument to
> regmap_bus to guard this chunking behavior?

I think this is just software definition difference whether to
include reg addr in max_raw_read/write or not, so no need to branch
the regmap implementation by a flag.

I'm a bit prefer to exclude the reg addr, as it is in stable tree now
and doesn't see strong reason to change it. And suggest regmap-i2c does
the same as spi do, that is to reserve space for reg addr/padding by
reducing the max_raw_read/write value, see:

https://lore.kernel.org/all/20220818104851.429479-1-cristian.ciocaltea@collabora.com/

Thanks,
Yilun

> 
> >
> > Please include human readable descriptions of things like commits and
> > issues being discussed in e-mail in your mails, this makes them much
> > easier for humans to read especially when they have no internet access.
> > I do frequently catch up on my mail on flights or while otherwise
> > travelling so this is even more pressing for me than just being about
> > making things a bit easier to read.