Re: [PATCH v2 4/4] serial: qcom_geni: Use devm_krealloc_array

From: Jonathan Cameron <jic23@kernel.org>
To: James Clark <james.clark@arm.com>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
	linux-kernel@vger.kernel.org, linux@roeck-us.net,
	michal.simek@amd.com, Jonathan Corbet <corbet@lwn.net>,
	Jean Delvare <jdelvare@suse.com>,
	Anand Ashok Dumbre <anand.ashok.dumbre@xilinx.com>,
	Lars-Peter Clausen <lars@metafoo.de>,
	Michal Simek <michal.simek@xilinx.com>,
	Andy Gross <agross@kernel.org>,
	Bjorn Andersson <andersson@kernel.org>,
	Konrad Dybcio <konrad.dybcio@linaro.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jirislaby@kernel.org>,
	linux-doc@vger.kernel.org, linux-hwmon@vger.kernel.org,
	linux-iio@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-arm-msm@vger.kernel.org, linux-serial@vger.kernel.org
Subject: Re: [PATCH v2 4/4] serial: qcom_geni: Use devm_krealloc_array
Date: Sat, 18 Mar 2023 17:34:21 +0000	[thread overview]
Message-ID: <20230318173402.20a4f60d@jic23-huawei> (raw)
In-Reply-To: <74d8b579-6ea8-d6f3-170f-ea13534b4565@arm.com>

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

> On 11/03/2023 19:18, Jonathan Cameron wrote:
> > On Thu,  9 Mar 2023 15:03:33 +0000
> > James Clark <james.clark@arm.com> wrote:
> >   
> >> Now that it exists, use it instead of doing the multiplication manually.
> >>
> >> Signed-off-by: James Clark <james.clark@arm.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;
> >>  	}  
> >   