linux-kernel - Re: [PATCH] i2c: mediatek: modify threshold passed to i2c_get_dma_safe_msg

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Message-ID: <20190215092500.6q3hqnoufyowiqaz@ninjato>
Date:   Fri, 15 Feb 2019 10:25:00 +0100
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Hsin-Yi Wang <hsinyi@...omium.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Matthias Brugger <matthias.bgg@...il.com>,
        Jun Gao <jun.gao@...iatek.com>,
        Ryder Lee <ryder.lee@...iatek.com>, linux-i2c@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i2c: mediatek: modify threshold passed to
 i2c_get_dma_safe_msg_buf()

On Fri, Feb 15, 2019 at 05:17:27PM +0800, Hsin-Yi Wang wrote:
> Ok, I can add a check in another patch. Should we return NULL pointer
> if msg->len is 0 or print out some warnings? Thanks.

No warning, msg->len == 0 is a valid setting. But interesting question:
I was about to say NULL, but your driver would assume ENOMEM there and
discard the message which is also not correct since msg->len == 0 is a
valid setting. So, should we just return msg->buf then? Will this work
with your driver? Can it handle zero-length transfers?

> On Fri, Feb 15, 2019 at 5:10 PM Wolfram Sang <wsa@...-dreams.de> wrote:
> >
> > On Fri, Feb 15, 2019 at 05:02:02PM +0800, Hsin-Yi Wang wrote:
> > > i2c_get_dma_safe_msg_buf() allocates space based on msg->len. If threshold is
> > > 0 and msg->len is also 0, function makes zero-length allocation, which returns
> > > a special ZERO_SIZE_PTR instead of a NULL pointer, and this will cause later
> > > code to fail. Modify the threshold to > 0 so the function returns NULL pointer.
> > >
> > > Fixes: fc66b39fe36a ("i2c: mediatek: Use DMA safe buffers for i2c transactions")
> > > Signed-off-by: Hsin-Yi Wang <hsinyi@...omium.org>
> >
> > Right. But we need to fix i2c_get_dma_safe_msg_buf(), so it will be
> > handled for all callers. Do you want to add a check if msg->len is 0 or
> > shall I? Both fine for me.
> >

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