linux-kernel - Re: [PATCH v2 1/3] platform/x86: intel_scu

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Message-ID: <042a0a04-06e2-f277-f666-15f748469d59@linux.intel.com>
Date: Mon, 21 Oct 2024 13:14:54 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
cc: LKML <linux-kernel@...r.kernel.org>, platform-driver-x86@...r.kernel.org, 
    Mika Westerberg <mika.westerberg@...ux.intel.com>, 
    Hans de Goede <hdegoede@...hat.com>, Ferry Toth <fntoth@...il.com>
Subject: Re: [PATCH v2 1/3] platform/x86: intel_scu_ipc: Replace workaround
 by 32-bit IO

On Mon, 21 Oct 2024, Andy Shevchenko wrote:

> On Mon, Oct 21, 2024 at 12:54:16PM +0300, Andy Shevchenko wrote:
> > On Mon, Oct 21, 2024 at 12:49:08PM +0300, Ilpo Järvinen wrote:
> > > On Mon, 21 Oct 2024, Andy Shevchenko wrote:
> > > > On Mon, Oct 21, 2024 at 12:24:57PM +0300, Ilpo Järvinen wrote:
> > > > > On Mon, 21 Oct 2024, Andy Shevchenko wrote:
> 
> ...
> 
> > > > > > +		for (nc = 0, offset = 0; nc < 4; nc++, offset += 4)
> > > > > > +			wbuf[nc] = ipc_data_readl(scu, offset);
> > > > > > +		memcpy(data, wbuf, count);
> > > > > 
> > > > > So do we actually need to read more than
> > > > > DIV_ROUND_UP(min(count, 16U), sizeof(u32))? Because that's the approach 
> > > > > used in intel_scu_ipc_dev_command_with_size() which you referred to.
> > > > 
> > > > I'm not sure I follow. We do IO for whole (16-bytes) buffer, but return only
> > > > asked _bytes_ to the user.
> > > 
> > > So always reading 16 bytes is not part of the old workaround? Because it 
> > > has a "lets read enough" feel.
> > 
> > Ah, now I got it! Yes, we may reduce the reads to just needed ones.
> > The idea is that we always have to perform 32-bit reads independently
> > on the amount of data we want.
> 
> Oh, looking at the code (*) it seems they are really messed up in the original
> with bytes vs. 32-bit words! Since the above has been tested, let me put this
> on TODO list to clarify this mess and run with another testing.
> 
> Sounds good to you?

Sure, I'm fine with taking the careful approach.

> *) the mythical comment about max 5 items for 20-byte buffer is worrying and
> now I know why,

Those functions with that comment seem to only be called from 
scu_reg_access() which error checks count > 4.

-- 
 i.