linux-kernel - Re: [PATCH v2 3/4] i2c: designware: Allow SMBus block reads up to 255 bytes in length

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Date:   Thu, 17 Sep 2020 16:12:48 -0700
From:   Sultan Alsawaf <sultan@...neltoast.com>
To:     Wolfram Sang <wsa@...nel.org>, linux-i2c@...r.kernel.org,
        jikos@...nel.org, aaron.ma@...onical.com, admin@...ma.net,
        andriy.shevchenko@...ux.intel.com, benjamin.tissoires@...hat.com,
        hdegoede@...hat.com, hn.chen@...dahitech.com,
        jarkko.nikula@...ux.intel.com, kai.heng.feng@...onical.com,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        mika.westerberg@...ux.intel.com, vicamo.yang@...onical.com
Subject: Re: [PATCH v2 3/4] i2c: designware: Allow SMBus block reads up to
 255 bytes in length

On Thu, Sep 17, 2020 at 10:57:04PM +0200, Wolfram Sang wrote:
> On Wed, Sep 16, 2020 at 10:22:55PM -0700, Sultan Alsawaf wrote:
> > From: Sultan Alsawaf <sultan@...neltoast.com>
> > 
> > According to the SMBus 3.0 protocol specification, block transfer limits
> > were increased from 32 bytes to 255 bytes. Remove the obsolete 32-byte
> > limitation.
> 
> Sadly, it is not that easy. We are trying to extend BLOCK_MAX to 255
> (SMBus 3 specs) but there are various things to be considered,
> especially with buffers and when passing it to userspace. Check here for
> the discussion (and you are welcome to join, of course):
> 
> http://patchwork.ozlabs.org/project/linux-i2c/list/?submitter=79741&state=*
> 
> > 
> > Signed-off-by: Sultan Alsawaf <sultan@...neltoast.com>
> > ---
> >  drivers/i2c/busses/i2c-designware-master.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/i2c/busses/i2c-designware-master.c b/drivers/i2c/busses/i2c-designware-master.c
> > index 22f28516bca7..5bd64bd17d94 100644
> > --- a/drivers/i2c/busses/i2c-designware-master.c
> > +++ b/drivers/i2c/busses/i2c-designware-master.c
> > @@ -433,7 +433,7 @@ i2c_dw_read(struct dw_i2c_dev *dev)
> >  			regmap_read(dev->map, DW_IC_DATA_CMD, &tmp);
> >  			if (flags & I2C_M_RECV_LEN) {
> >  				/* Ensure length byte is a valid value */
> > -				if (tmp <= I2C_SMBUS_BLOCK_MAX && tmp > 0)
> > +				if (tmp > 0)
> >  					len = i2c_dw_recv_len(dev, tmp);
> >  				else
> >  					len = i2c_dw_recv_len(dev, len);
> > -- 
> > 2.28.0
> > 

Yes, it is not that easy to make the change on a global scale. However, in the
case of the designware adapter, it really *is* that easy. This change covers the
designware adapter, and others can follow later with the much more invasive
changes that are needed.

Sultan