linux-kernel - Re: drivers/gpu/drm/bridge/fsl-ldb.c:101: possible loss of information.

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Message-ID: <20230309100415.GO31765@pendragon.ideasonboard.com>
Date:   Thu, 9 Mar 2023 12:04:15 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     David Binderman <dcb314@...mail.com>
Cc:     "andrzej.hajda@...el.com" <andrzej.hajda@...el.com>,
        "neil.armstrong@...aro.org" <neil.armstrong@...aro.org>,
        "rfoss@...nel.org" <rfoss@...nel.org>,
        "jonas@...boo.se" <jonas@...boo.se>,
        "jernej.skrabec@...il.com" <jernej.skrabec@...il.com>,
        "airlied@...il.com" <airlied@...il.com>,
        "daniel@...ll.ch" <daniel@...ll.ch>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: drivers/gpu/drm/bridge/fsl-ldb.c:101: possible loss of
 information.

Hi David,

On Thu, Mar 09, 2023 at 09:42:54AM +0000, David Binderman wrote:
> Hello there Laurent,
> 
> > Would you be able to send a patch to fix this ?
> 
> Sadly, no. My success rate with kernel patches is low enough to make
> it not worth trying.

I'm sorry to hear that. If you were willing to try again, I can offer
help with tooling and review to get your patch merged.

> From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> Sent: 09 March 2023 09:26
> To: David Binderman <dcb314@...mail.com>
> Cc: andrzej.hajda@...el.com <andrzej.hajda@...el.com>; neil.armstrong@...aro.org <neil.armstrong@...aro.org>; rfoss@...nel.org <rfoss@...nel.org>; jonas@...boo.se <jonas@...boo.se>; jernej.skrabec@...il.com <jernej.skrabec@...il.com>; airlied@...il.com <airlied@...il.com>; daniel@...ll.ch <daniel@...ll.ch>; dri-devel@...ts.freedesktop.org <dri-devel@...ts.freedesktop.org>; linux-kernel@...r.kernel.org <linux-kernel@...r.kernel.org>
> Subject: Re: drivers/gpu/drm/bridge/fsl-ldb.c:101: possible loss of information. 
>  
> Hi David,
> 
> On Thu, Mar 09, 2023 at 07:59:34AM +0000, David Binderman wrote:
> > Hello there Laurent,
> > 
> > > We could, but I don't think it will make any difference in practice as
> > > the maximum pixel clock frequency supported by the SoC is 80MHz (per
> > > LVDS channel). That would result in a 560MHz frequency returned by this
> > > function, well below the 31 bits limit.
> > 
> > Thanks for your explanation. I have a couple of suggestions for possible improvements:
> > 
> > 1. Your explanatory text in a comment nearby. This helps all readers of the code.
> > 
> > 2. Might the frequency go up to 300 MHz anytime soon ? The code will stop working then. 
> > In this case, I would suggest to put in a run time sanity check to make sure no 31 bit overflow. 
> 
> As it's a hardware limit of the SoC, I wouldn't expect so.
> 
> This being said, I think adding a UL suffix to the constants would be
> better than a comment as it will please static checkers and serve as
> documentation to humans. Would you be able to send a patch to fix this ?
> 
> > Just a couple of ideas for the code.
> 
> Thanks for taking the time to share those.

-- 
Regards,

Laurent Pinchart