RE: [RFC PATCH 1/9] apple-gmux: use cpu_to_be32 instead of manual reorder

[David Laight]

From: Hans de Goede
> Sent: 10 February 2023 19:33
> 
> Hi,
> 
> On 2/10/23 20:09, Hans de Goede wrote:
> > Hi,
> >
> > On 2/10/23 05:48, Orlando Chamberlain wrote:
> >> Currently it manually flips the byte order, but we can instead use
> >> cpu_to_be32(val) for this.
> >>
> >> Signed-off-by: Orlando Chamberlain <orlandoch.dev@xxxxxxxxx>
> >> ---
> >>  drivers/platform/x86/apple-gmux.c | 18 ++----------------
> >>  1 file changed, 2 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/drivers/platform/x86/apple-gmux.c b/drivers/platform/x86/apple-gmux.c
> >> index 9333f82cfa8a..e8cb084cb81f 100644
> >> --- a/drivers/platform/x86/apple-gmux.c
> >> +++ b/drivers/platform/x86/apple-gmux.c
> >> @@ -94,13 +94,7 @@ static u32 gmux_pio_read32(struct apple_gmux_data *gmux_data, int port)
> >>  static void gmux_pio_write32(struct apple_gmux_data *gmux_data, int port,
> >>  			     u32 val)
> >>  {
> >> -	int i;
> >> -	u8 tmpval;
> >> -
> >> -	for (i = 0; i < 4; i++) {
> >> -		tmpval = (val >> (i * 8)) & 0xff;
> >> -		outb(tmpval, gmux_data->iostart + port + i);
> >> -	}
> >> +	outl(cpu_to_be32(val), gmux_data->iostart + port);
> >>  }
> >>
> >>  static int gmux_index_wait_ready(struct apple_gmux_data *gmux_data)
> >
> > The ioport / indexed-ioport accessed apple_gmux-es likely are (part of?)
> > LPC bus devices . Looking at the bus level you are now changing 4 io
> > accesses with a size of 1 byte, to 1 32 bit io-access.
> 
> Correction to myself, re-reading the LPC specification, then
> if I'm right and this is a LPC device then all IO in/out accesses
> are always 1 byte accesses. Since the LPC bus only supports 16 / 32
> bit accesses for DMA cycles.
> 
> So presumably the outl() would get split into 4 separate 8 bit
> (port) IO accesses.

I wonder if there is something obscure and the order of the
4 bytes writes matters?

In any case writing as:
	xxxx iostart = gmux_data->iostart + port;

	outb(val, iostart);
	outb(val >> 8, iostart + 1);
	outb(val >> 16, iostart + 2);
	outb(val >> 24, ioctart + 3);
almost certainly generates better code.

	David

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