linux-kernel - Re: [PATCH] ACPI: SPCR: Use access width to determine mmio usage

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Message-ID: <f122adfb-872e-9ee6-db61-b55c75903b63@redhat.com>
Date:   Mon, 8 May 2017 15:50:05 -0400
From:   Jon Masters <jcm@...hat.com>
To:     Loc Ho <lho@....com>
Cc:     Jon Mason <jon.mason@...adcom.com>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Robert Moore <robert.moore@...el.com>,
        Lv Zheng <lv.zheng@...el.com>, linux-acpi@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        devel@...ica.org, bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH] ACPI: SPCR: Use access width to determine mmio usage

On 05/08/2017 03:11 PM, Loc Ho wrote:
> Hi Jon,
> 
>>> The current SPCR code does not check the access width of the mmio, and
>>> uses a default of 8bit register accesses.  This prevents devices that
>>> only do 16 or 32bit register accesses from working.  By simply checking
>>> this field and setting the mmio string appropriately, this issue can be
>>> corrected.  To prevent any legacy issues, the code will default to 8bit
>>> accesses if the value is anything but 16 or 32.
>>
>> Thanks for this. Just as an FYI I've a running discussion with Microsoft
>> about defining additional UART subtypes in the DBG2 for special case
>> UARTs. Specifically, I want to address AppliedMicro's special 8250 dw IP
>> that also has a non-standard clock. At this time, there is general
>> agreement to use the access width for some cases rather than defining
>> yet more subtypes - so your patch is good.
>>
>> Loc/Applied: please track this thread, incorporate feedback, and also
>> track the other general recent discussions of 8250 dw from this week.
> 
> Thanks for forward me this patch. This patch does not work with X-Gene
> v1 and v2 SoC's. As BIOS SPCR encodes these info as:
> 
> Bit Width: 32
> Bit Offset: 0
> Encoded Access Width: 01 (Byte Access)
> 
> With this patch, it would use the "mmio" instead the "mmio32" as with
> this patch - https://patchwork.kernel.org/patch/9460959

I think this is why we need the DBG2 subtype for Applied X-Gene1. I'm
hoping the update to the SPCR/DBG2 spec is done soon.

Jon.

-- 
Computer Architect | Sent from my Fedora powered laptop