linux-kernel - Re: [PATCH 0/2] soundwire: Remove redundant zeroing of page registers

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Message-ID: <08b9871d-54fa-bbef-a5ca-0be888312645@linux.intel.com>
Date:   Fri, 2 Dec 2022 10:45:38 -0600
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Richard Fitzgerald <rf@...nsource.cirrus.com>, vkoul@...nel.org
Cc:     alsa-devel@...a-project.org, patches@...nsource.cirrus.com,
        linux-kernel@...r.kernel.org, sanyog.r.kale@...el.com,
        yung-chuan.liao@...ux.intel.com
Subject: Re: [PATCH 0/2] soundwire: Remove redundant zeroing of page registers



On 12/2/22 05:26, Richard Fitzgerald wrote:
> On 01/12/2022 18:31, Pierre-Louis Bossart wrote:
>>
>>
>> On 12/1/22 08:08, Richard Fitzgerald wrote:
>>> Writing zero to the page registers after each message transaction can
>>> add
>>> up to a lot of overhead for codecs that need to transfer large amount of
>>> data - for example a firmware download.
>>>
>>> There's no spec reason I can see for this zeroing. The page registers
>>> are
>>> only used for a paged address. The bus code uses a non-paged address for
>>> registers in page 0. It always writes the page registers at the start of
>>> a paged transaction.
>>>
>>> If this zeroing was a workaround for anything, let me know and I will
>>> re-implement the zeroing as a quirk that can be enabled only when it is
>>> necessary.
>>
>> It's a feature, not a bug :-)
>>
>> The page registers have to be zeroed out so that any bus-management
>> command hits the page0 instead of using a value that was set by codec
>> driver for vendor-specific configurations.
>>
> 
> Why would these bus management commands set bit 15 to indicate a paged
> access? If they don't set bit 15 the page registers are not used and
> bits 15..31 of the register address must be 0. Table 78 in the Soundwire
> 1.2 spec. Table 71 in the 1.0 spec. Table 43 in the 0.6 draft spec.

I forgot about this magic BIT(15) and indeed the Addr_page1/2 values are
ignored when issuing non-paged register access. There's really no need
to zero-out the page registers, it's completely unnecessary. Nice catch!

For the series:

Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>



>> The implementation is far from optimal though, and indeed if we have
>> long transactions that are not interrupted by anything else we could
>> avoid resetting the page registers.
>>
>> I tried to implement a 'lazy approach' some time back, but at the time I
>> didn't see any benefits due to the limited number of configurations.
>>
>> I can't remember where the code is, but the initial enhancement was
>> listed here: https://github.com/thesofproject/linux/issues/2881
>>
>>>
>>> Richard Fitzgerald (2):
>>>    soundwire: bus: Don't zero page registers after every transaction
>>>    soundwire: bus: Remove unused reset_page_addr() callback
>>>
>>>   drivers/soundwire/bus.c             | 23 -----------------------
>>>   drivers/soundwire/cadence_master.c  | 14 --------------
>>>   drivers/soundwire/cadence_master.h  |  3 ---
>>>   drivers/soundwire/intel_auxdevice.c |  1 -
>>>   include/linux/soundwire/sdw.h       |  3 ---
>>>   5 files changed, 44 deletions(-)
>>>