Re: [PATCH v5 1/5] Implement an ioctl to support the USMTMC-USB488 READ_STATUS_BYTE operation.

[Dave Penkler]

On Sun, Nov 22, 2015 at 12:32:41PM +0200, Andy Shevchenko wrote:
> On Sun, Nov 22, 2015 at 11:19 AM, Dave Penkler <dpenkler@xxxxxxxxx> wrote:
> > On Wed, Nov 18, 2015 at 11:55:27AM +0200, Andy Shevchenko wrote:
> >> On Wed, Nov 18, 2015 at 10:37 AM, Dave Penkler <dpenkler@xxxxxxxxx> wrote:
> 
> >> > +       switch (status) {
> >> > +       case 0: /* SUCCESS */
> >> > +               if (data->iin_buffer[0] & 0x80) {
> >> > +                       /* check for valid STB notification */
> >> > +                       if ((data->iin_buffer[0] & 0x7f) > 1) {
> >>
> >> Despite your answer to my comment code is quite understandable even with & 0x7e.
> >> You already put comment line about this, you may add that you validate
> >> the value to be 127 >= value >= 2.
> >>
> >
> > Yes it is quite understandable but it is less clear. I repeat my comment here:
> > When reading the spec and the code it is more obvious that here
> > we are testing for the value in bits D6..D0 to be a valid iin_bTag return.
> > (See Table 7 in the USBTMC-USB488 spec.)
> >
> > What is your motivation for
> >
> >  if (data->iin_buffer[0] & 0x7e)
> >
> > ?
> 
> In non-optimized variant it will certainly generate less code. You may
> have check assembly code with -O2 and compare. I don't know if
> compiler is clever enough to do the same by itself.
> 

I tested out both variants, and the explicit test is actually faster on by box:

$ cat tp.c
#include <stdlib.h>
#include <stdio.h>
#define xstr(s) str(s)
#define str(s) #s
main() {
        unsigned int v,s=0;
        struct recs {
                unsigned char *iin_buffer;
        } rec;
        struct recs *data = &rec;
        data->iin_buffer = (unsigned char *) malloc(8);
        for (v=1;v;v++) {
                data->iin_buffer[0] = v & 0x7f;
                if (TEST)
                        s++;
        }
        printf("%s %x\n",xstr(TEST),s);
}
$ cc -O2 tp.c -DTEST='data->iin_buffer[0] & 0x7e'
$ time ./a.out
data->iin_buffer[0] & 0x7e fc000000

real    0m3.927s
user    0m3.920s
sys     0m0.000s
$ time ./a.out
data->iin_buffer[0] & 0x7e fc000000

real    0m3.925s
user    0m3.920s
sys     0m0.000s
$ cc -O2 tp.c -DTEST='(data->iin_buffer[0] & 0x7f) > 1'
$ time ./a.out
(data->iin_buffer[0] & 0x7f) > 1 fc000000

real    0m2.638s
user    0m2.610s
sys     0m0.000s
$ time ./a.out
(data->iin_buffer[0] & 0x7f) > 1 fc000000

real    0m2.648s
user    0m2.620s
sys     0m0.000s

> >> > +               /* urb terminated, clean up */
> >> > +               dev_dbg(&data->intf->dev,
> >> > +                       "%s - urb terminated, status: %d\n",
> >> > +                       __func__, status);
> >>
> >> No need to print function here explicitly. Check Dynamic Debug framework.
> >
> > I am not using dynamic debug but when I enable static debug I get:
> >
> > [ 1438.562458] usbtmc 1-1:1.0: Enter ioctl_read_stb iin_ep_present: 1
> >
> > on the console log for
> >
> > dev_dbg(dev, "Enter ioctl_read_stb iin_ep_present: %d\n",
> >                 data->iin_ep_present);
> >
> > So if I don't print the function it does not appear on the log.
> 
> Whatever maintainers prefer, though I think there are quite rare cases
> in USB when someone needs static debug. I'm pretty sure most of the
> developers all in favour of dynamic debug.
> 

I am happy to remove the func

> >> >         retcode = sysfs_create_group(&intf->dev.kobj, &data_attr_grp);
> >> >
> >> >         retcode = usb_register_dev(intf, &usbtmc_class);
> >>
> >> Hmm??? Unrelated to this patch, but notice that retcode is overridden here.
> 
> 
> 
> -- 
> With Best Regards,
> Andy Shevchenko
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