linux-kernel - RE: [PATCH 3/8] thunderbolt: Use 32-bit writes when writing ring producer/consumer

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Message-ID: <79616dd147864771b0b74901e77f2607@AcuMS.aculab.com>
Date:   Wed, 7 Aug 2019 16:22:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mika Westerberg' <mika.westerberg@...ux.intel.com>
CC:     'Yehezkel Bernat' <yehezkelshb@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andreas Noever <andreas.noever@...il.com>,
        Michael Jamet <michael.jamet@...el.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, Lukas Wunner <lukas@...ner.de>,
        Mario Limonciello <Mario.Limonciello@...l.com>,
        "Anthony Wong" <anthony.wong@...onical.com>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
Subject: RE: [PATCH 3/8] thunderbolt: Use 32-bit writes when writing ring
 producer/consumer

From: Mika Westerberg
> Sent: 07 August 2019 17:14
> To: David Laight
> 
> On Fri, Jul 05, 2019 at 04:04:19PM +0000, David Laight wrote:
> > > Really a matter of taste, but maybe you want to consider having a single
> > > function, with a 3rd parameter, bool is_tx.
> > > The calls here will be unified to:
> > >         ring_iowrite(ring, ring->head, ring->is_tx);
> > > (No condition is needed here).
> > >
> > > The implementation uses the new parameter to decide which part of the register
> > > to mask, reducing the code duplication (in my eyes):
> > >
> > >         val = ioread32(ring_desc_base(ring) + 8);
> > >         if (is_tx) {
> > >                 val &= 0x0000ffff;
> > >                 val |= value << 16;
> > >         } else {
> > >                 val &= 0xffff0000;
> > >                 val |= value;
> > >         }
> > >         iowrite32(val, ring_desc_base(ring) + 8);
> > >
> > > I'm not sure if it improves the readability or makes it worse. Your call.
> >
> > Gah, that is all horrid beyond belief.
> > If a 32bit write is valid then the hardware must not be updating
> > the other 16 bits.
> > In which case the driver knows what they should be.
> > So it can do a single 32bit write of the required value.
> 
> I'm not entirely sure I understand what you say above. Can you shed some
> light on this by a concrete example how it should look like? :-)

The driver must know both the tx and rx ring values, so:
	iowrite32(tx_val << 16 | rx_val, ring_desc_base(ring) + 8);

The ioread32() is likely to be very slow - you only want to do them
if absolutely necessary.
The speed of the iowrite32() doesn't matter (much) since it is almost
certainly 'posted' and execution continues while the bus cycle is
in progress.

	David

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