linux-kernel - Re: [PATCH v2 1/2] acpi, spcr: Make SPCR avialable to other architectures

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Date:   Wed, 13 Dec 2017 15:11:33 -0600
From:   Timur Tabi <timur@...eaurora.org>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Prarit Bhargava <prarit@...hat.com>
Cc:     linux-acpi@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org, linux-serial@...r.kernel.org,
        Bhupesh Sharma <bhsharma@...hat.com>,
        Lv Zheng <lv.zheng@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, graeme.gregory@...aro.org,
        mark.salter@...hat.com
Subject: Re: [PATCH v2 1/2] acpi, spcr: Make SPCR avialable to other
 architectures

On 12/13/2017 06:45 AM, Lorenzo Pieralisi wrote:
>> +/*
>> + * Erratum 44 for QDF2432v1 and QDF2400v1 SoCs describes the BUSY bit as
>> + * occasionally getting stuck as 1. To avoid the potential for a hang, check
>> + * TXFE == 0 instead of BUSY == 1. This may not be suitable for all UART
>> + * implementations, so only do so if an affected platform is detected in
>> + * acpi_parse_spcr().
>> + */
>> +bool qdf2400_e44_present;
>> +EXPORT_SYMBOL(qdf2400_e44_present);
> 
> My eyes, this is horrible but it is not introduced by this patch. It
> would have been much better if:
> 
> drivers/tty/serial/amba-pl011.c
> 
> parsed the SPCR table (again) to detect it instead of relying on this
> horrible exported flag.

I didn't want to put any ACPI code in amba-pl011.c, so putting it in 
spcr.c made the most sense.  I agree the global variable is ugly.  If 
you have a better idea, I'm all ears.

If it's any consolation, this erratum affects only 1.x silicon, which is 
technically pre-production (although a lot of people have them).  This 
work-around will eventually be reverted.

>> diff --git a/drivers/acpi/Kconfig b/drivers/acpi/Kconfig
>> index 46505396869e..9ae98eeada76 100644
>> --- a/drivers/acpi/Kconfig
>> +++ b/drivers/acpi/Kconfig
>> @@ -79,7 +79,12 @@ config ACPI_DEBUGGER_USER
>>   endif
>>   
>>   config ACPI_SPCR_TABLE
>> -	bool
>> +	bool "ACPI Serial Port Console Redirection Support"
>> +	default y if ARM64
> 
> You need to remove the selection in arch/arm64 then. Also, moving away
> from a non-visible config may have consequences on ARM64, Graeme and
> Mark are more familiar with the SPCR dependencies so please chime in.

I did raise this as a concern in the previous version of the patch.  I 
also think it should not be a selectable option.

Keeping the "select" does force SPCR to be enabled on ARM64 ACPI 
platforms, but if it's an option for x86, it should be an option for ARM.

-- 
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