linux-kernel - Re: [PATCH 4/4] serial: 8250_dw: Rework ->serial

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Date:   Wed, 29 Jun 2022 12:40:01 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
cc:     "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] serial: 8250_dw: Rework ->serial_out() LCR write
 retry logic

On Wed, 29 Jun 2022, Andy Shevchenko wrote:

> On Wed, Jun 29, 2022 at 10:47 AM Ilpo Järvinen
> <ilpo.jarvinen@...ux.intel.com> wrote:
> > On Tue, 28 Jun 2022, Andy Shevchenko wrote:
> > > On Tue, Jun 28, 2022 at 3:43 PM Ilpo Järvinen
> > > <ilpo.jarvinen@...ux.intel.com> wrote:
> > > >
> > > > Currently dw8250_verify_write() (was dw8250_check_lcr()) nullifies the
> > > > benefit from differentiated ->serial_out() by having big if tree to
> > > > select correct write type.
> > > >
> > > > Rework the logic such that the LCR write can be retried within the
> > > > relevant ->serial_out() handler:
> > > >   1. Move retries counter on the caller level and pass as pointer to
> > > >      dw8250_verify_write()
> > > >   2. Make dw8250_verify_write() return bool
> > > >   3. Retry the write on caller level (if needed)
> > >
> > > I'm wondering if it's possible to utilize one of iopoll.h macro here
> > > instead of copying retries and that not-so-obvious IO poll write.
> >
> > Eh, are you suggesting I should do write as a side-effect inside one of
> > the iopoll.h macros? Because those available seem to only read?
> >
> > Or should I create another macro there which writes too?
> 
> It seems to me that it would be a macro on top of iopoll's one which
> will take an op read and op write arguments depending on the case.

The thing is those iopoll macros don't return until the timeout is 
exhausted so I don't think I can reuse them easily for this task ("on top 
of iopoll's one")? That is, w/o some major side-effect hack (which is 
IMHO a no-go).

-- 
 i.

> Note, for that special case you would need a custom write op instead
> of simple __raw_writeq().
>
> Try and if it looks better, convert, otherwise it would be nice to
> hear why it won't fly in your opinion.