linux-kernel - Re: [PATCH v3 10/24] drm/rockchip: dw-mipi-dsi: don't assume buffer is aligned

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Message-ID: <20170131115618.633398fc.john@metanate.com>
Date:   Tue, 31 Jan 2017 11:56:18 +0000
From:   John Keeping <john@...anate.com>
To:     Sean Paul <seanpaul@...omium.org>
Cc:     Mark Yao <mark.yao@...k-chips.com>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        linux-rockchip@...ts.infradead.org,
        Chris Zhong <zyw@...k-chips.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 10/24] drm/rockchip: dw-mipi-dsi: don't assume buffer
 is aligned

On Mon, 30 Jan 2017 15:08:11 -0500, Sean Paul wrote:

> On Sun, Jan 29, 2017 at 01:24:30PM +0000, John Keeping wrote:
> > By dereferencing the MIPI command buffer as a u32* we rely on it being
> > correctly aligned on ARM, but this may not be the case.  Copy it into a
> > stack variable that will be correctly aligned.
> > 
> > Signed-off-by: John Keeping <john@...anate.com>
> > Reviewed-by: Chris Zhong <zyw@...k-chips.com>
> > ---
> > v3:
> > - Add Chris' Reviewed-by
> > Unchanged in v2
> > 
> >  drivers/gpu/drm/rockchip/dw-mipi-dsi.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > index 03fc096fe1bd..ddbc037e7ced 100644
> > --- a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > +++ b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > @@ -607,10 +607,10 @@ static int dw_mipi_dsi_dcs_short_write(struct dw_mipi_dsi *dsi,
> >  static int dw_mipi_dsi_dcs_long_write(struct dw_mipi_dsi *dsi,
> >  				      const struct mipi_dsi_msg *msg)
> >  {
> > -	const u32 *tx_buf = msg->tx_buf;
> > -	int len = msg->tx_len, pld_data_bytes = sizeof(*tx_buf), ret;
> > +	const u8 *tx_buf = msg->tx_buf;
> > +	int len = msg->tx_len, pld_data_bytes = sizeof(u32), ret;
> >  	u32 hdr_val = GEN_HDATA(msg->tx_len) | GEN_HTYPE(msg->type);
> > -	u32 remainder = 0;
> > +	u32 remainder;
> >  	u32 val;
> >  
> >  	if (msg->tx_len < 3) {
> > @@ -621,12 +621,14 @@ static int dw_mipi_dsi_dcs_long_write(struct dw_mipi_dsi *dsi,
> >  
> >  	while (DIV_ROUND_UP(len, pld_data_bytes)) {
> >  		if (len < pld_data_bytes) {
> > +			remainder = 0;
> >  			memcpy(&remainder, tx_buf, len);
> >  			dsi_write(dsi, DSI_GEN_PLD_DATA, remainder);
> >  			len = 0;
> >  		} else {
> > -			dsi_write(dsi, DSI_GEN_PLD_DATA, *tx_buf);
> > -			tx_buf++;
> > +			memcpy(&remainder, tx_buf, pld_data_bytes);
> > +			dsi_write(dsi, DSI_GEN_PLD_DATA, remainder);
> > +			tx_buf += pld_data_bytes;
> >  			len -= pld_data_bytes;
> >  		}  
> 
> 
> You can clean this up further by removing a couple of the locals, the
> conditional and the division:
> 
> while(len < msg->tx_len) {
>         size_t to_write = MIN(msg->tx_len - len, sizeof(remainder));
> 
>         memcpy(&remainder, msg->tx_buf + len, to_write);
>         dsi_write(dsi, DSI_GEN_PLD_DATA, remainder);
>         len += to_write;
> 
>         ...
> }

That's a nice simplification, but it's more than I was trying to do with
this patch.  I can add a cleanup patch on top to simplify the function,
but I don't have that much time to spend on this at the moment and I'd
rather not block this fix because the original code structure could be
improved.