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

Jonathan Cameron
2023-03-18T17:19:39Z

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;
> >> }
> >