linux-kernel - Re: [PATCH v2 4/4] serial: qcom_geni: Use devm_krealloc

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Message-ID: <20230318173402.20a4f60d@jic23-huawei>
Date:   Sat, 18 Mar 2023 17:34:21 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     James Clark <james.clark@....com>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        linux-kernel@...r.kernel.org, linux@...ck-us.net,
        michal.simek@....com, Jonathan Corbet <corbet@....net>,
        Jean Delvare <jdelvare@...e.com>,
        Anand Ashok Dumbre <anand.ashok.dumbre@...inx.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Michal Simek <michal.simek@...inx.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>, linux-doc@...r.kernel.org,
        linux-hwmon@...r.kernel.org, linux-iio@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v2 4/4] serial: qcom_geni: Use devm_krealloc_array

On Fri, 17 Mar 2023 11:34:49 +0000
James Clark <james.clark@....com> wrote:

> On 11/03/2023 19:18, Jonathan Cameron wrote:
> > On Thu,  9 Mar 2023 15:03:33 +0000
> > James Clark <james.clark@....com> wrote:
> >   
> >> Now that it exists, use it instead of doing the multiplication manually.
> >>
> >> Signed-off-by: James Clark <james.clark@....com>  
> > 
> > Hmm. I've stared at the users of this for a bit, and it's not actually obvious
> > that it's being used as an array of u32.  The only typed user of this is as
> > the 2nd parameter of  
> > tty_insert_flip_string() which is an unsigned char *
> > 
> > I wonder if that sizeof(u32) isn't a 'correct' description of where the 4 is coming
> > from even if it has the right value?  Perhaps the fifo depth is just a multiple of 4?
> > 
> > Jonathan
> >   
> 
> The commit that added it (b8caf69a6946) seems to hint that something
> reads from it in words. And I see this:
> 
>   /* We always configure 4 bytes per FIFO word */
>   #define BYTES_PER_FIFO_WORD		4U
> 
> Perhaps sizeof(u32) isn't as accurate of a description as using
> BYTES_PER_FIFO_WORD but I'd be reluctant to make a change because I
> don't really understand the implications.

Agreed with your analysis.  + fully understand why you don't want to change
it. 

I'd be tempted to take the view that whilst it's allocated in 4 byte chunks
because it's accessed elsewhere as a set of 1 byte entries, krealloc_array
isn't appropriate and so just leave it with devm_krealloc()

Risk is that a steady stream of patches will turn up 'fixing' this as
it will be easy for people to find with a script.  Maybe better to just add
a comment (either with or without your patch).
> 
> There is also this in handle_rx_console():
> 
>   unsigned char buf[sizeof(u32)];
> 
> James
> 
> > 
> >   
> >> ---
> >>  drivers/tty/serial/qcom_geni_serial.c | 6 +++---
> >>  1 file changed, 3 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> >> index d69592e5e2ec..23fc33d182ac 100644
> >> --- a/drivers/tty/serial/qcom_geni_serial.c
> >> +++ b/drivers/tty/serial/qcom_geni_serial.c
> >> @@ -1056,9 +1056,9 @@ static int setup_fifos(struct qcom_geni_serial_port *port)
> >>  		(port->tx_fifo_depth * port->tx_fifo_width) / BITS_PER_BYTE;
> >>  
> >>  	if (port->rx_buf && (old_rx_fifo_depth != port->rx_fifo_depth) && port->rx_fifo_depth) {
> >> -		port->rx_buf = devm_krealloc(uport->dev, port->rx_buf,
> >> -					     port->rx_fifo_depth * sizeof(u32),
> >> -					     GFP_KERNEL);
> >> +		port->rx_buf = devm_krealloc_array(uport->dev, port->rx_buf,
> >> +						   port->rx_fifo_depth, sizeof(u32),
> >> +						   GFP_KERNEL);
> >>  		if (!port->rx_buf)
> >>  			return -ENOMEM;
> >>  	}  
> >