linux-kernel - Re: [PATCH] xhci: Use ffs() to find page size in xhci_mem

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Message-ID: <CAHQ1cqE-GeW_03edN4u_-RsZ9bm2_S07foY3obYreHp-Efpbdg@mail.gmail.com>
Date:   Thu, 7 Feb 2019 13:06:36 -0800
From:   Andrey Smirnov <andrew.smirnov@...il.com>
To:     Mathias Nyman <mathias.nyman@...ux.intel.com>
Cc:     linux-usb@...r.kernel.org, Mathias Nyman <mathias.nyman@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xhci: Use ffs() to find page size in xhci_mem_init()

On Thu, Feb 7, 2019 at 1:00 AM Mathias Nyman
<mathias.nyman@...ux.intel.com> wrote:
>
> On 07.02.2019 02:03, Andrey Smirnov wrote:
> > Get page size order using ffs() instead of open coding it with a loop.
> >
> > Signed-off-by: Andrey Smirnov <andrew.smirnov@...il.com>
> > Cc: Mathias Nyman <mathias.nyman@...el.com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: linux-usb@...r.kernel.org
> > Cc: linux-kernel@...r.kernel.org
> > ---
> >   drivers/usb/host/xhci-mem.c | 6 +-----
> >   1 file changed, 1 insertion(+), 5 deletions(-)
> >
> > diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
> > index 36a3eb8849f1..44b43c3d819f 100644
> > --- a/drivers/usb/host/xhci-mem.c
> > +++ b/drivers/usb/host/xhci-mem.c
> > @@ -2362,11 +2362,7 @@ int xhci_mem_init(struct xhci_hcd *xhci, gfp_t flags)
> >       page_size = readl(&xhci->op_regs->page_size);
> >       xhci_dbg_trace(xhci, trace_xhci_dbg_init,
> >                       "Supported page size register = 0x%x", page_size);
> > -     for (i = 0; i < 16; i++) {
> > -             if ((0x1 & page_size) != 0)
> > -                     break;
> > -             page_size = page_size >> 1;
> > -     }
> > +     i = ffs(page_size);
> >       if (i < 16)
> >               xhci_dbg_trace(xhci, trace_xhci_dbg_init,
> >                       "Supported page size of %iK", (1 << (i+12)) / 1024);
>
> Hi
>
> using ffs() is a welcome change, but it will give different a result than the loop.
>
> *old loop
>    valid page_size value if i < 16
> *ffs()
>    valid page_size value if i >= 1 and i < 17
>

My bad, sorry about that, was using __fls() in the version I tested,
but then switched it to ffs() at the last minute and forgot to double
check. Will update in v2.

Thanks,
Andrey Smirnov