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

[Jon Mason]

On Tue, Jun 20, 2017 at 2:33 PM, Loc Ho <lho@xxxxxxx> 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.
>> >> >>
>> >> >> We can't rely on the BIOS change to support this new subtype as we
>> >> >> have system that is already in production deployment. When these
>> >> >> system upgrade to new version of the OS (stock, RHELSA, or whatever),
>> >> >> they will break. We need the patch from
>> >> >> https://patchwork.kernel.org/patch/9460959/ rolled upstream.
>> >> >
>> >> > There is no reason why the patch you reference cannot co-exist with
>> >> > the one I am submitting here.  In this case, my patch would set it to
>> >> > mmio, then the patch you link above would reset it to mmio32.
>> >> > Personally, I would recommend a big, fat comment on why this extra
>> >> > step is necessary, but it should work as desired.  Alternatively, we
>> >> > could add some kind of quirk library (similar to
>> >> > qdf2400_erratum_44_present) where the OEM/OEM Table ID is referenced
>> >> > and workaround applied.  Thoughts?
>> >>
>> >> That's was my first version but after seeing both versions, I think
>> >> they are better solution as it works for more SoC's than just our. As
>> >> you had suggested, we should apply your patch and
>> >> https://patchwork.kernel.org/patch/9460959. The third patch -
>> >> https://patchwork.kernel.org/patch/9462183/ - conflicts with your.
>> >>
>> >> Summary:
>> >> 1. Applied your - https://lkml.org/lkml/2017/5/4/450
>> >> 2. Applied this one - https://patchwork.kernel.org/patch/9460959/
>> >>
>> >> -Loc
>> >
>> > What if we simply applied the following (100% untested) patch to add
>> > the quirk framework I was suggesting?  It can be applied on top of the
>> > patch I submitted previously.
>>
>> It is a bit more complex that this simple patch. How about this one
>> (my original version). As for Jon Master question on McDivitt, not
>> sure what they use for the ACPI table for SPCR. If they used our
>> reference, then this might work for them too. This version would limit
>> to just the existent firmware or until the SPCR table gets changed.
>>
>>
>> tty: 8250: Workaround for APM X-Gene 8250 UART 32-alignment errata
>>
>> APM X-Gene verion 1 and 2 have an 8250 UART with its register
>> aligned to 32-bit. The SPCR always assumes fully compatible
>> 8250. This causes no console with ACPI boot as the console
>> will not match X-Gene UART port due to the lack of mmio32
>> option.
>>
>> Signed-off-by: Loc Ho <lho@xxxxxxx>
>> ---
>>  drivers/acpi/spcr.c | 21 +++++++++++++++++++++
>>  1 file changed, 21 insertions(+)
>>
>> diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
>> index 3afa8c1..77b45a0 100644
>> --- a/drivers/acpi/spcr.c
>> +++ b/drivers/acpi/spcr.c
>> @@ -36,6 +36,25 @@ static bool qdf2400_erratum_44_present(struct
>> acpi_table_header *h)
>>         return false;
>>  }
>>
>> +/*
>> + * APM X-Gene v1 and v2 UART hardware is an 16550 like device but has its
>> + * register aligned to 32-bit. This function detects this errata condition.
>> + */
>> +static bool xgene_8250_erratum_present(struct acpi_table_spcr *tb)
>> +{
>> +       if (tb->interface_type != ACPI_DBG2_16550_COMPATIBLE)
>> +               return false;
>> +
>> +       if (memcmp(tb->header.oem_id, "APMC0D", ACPI_OEM_ID_SIZE))
>> +               return false;
>> +
>> +       if (!memcmp(tb->header.oem_table_id, "XGENESPC",
>> +                   ACPI_OEM_TABLE_ID_SIZE) && tb->header.oem_revision == 0)
>> +               return true;
>> +
>> +       return false;
>> +}
>> +
>>  /**
>>   * parse_spcr() - parse ACPI SPCR table and add preferred console
>>   *
>> @@ -115,6 +134,8 @@ int __init parse_spcr(bool earlycon)
>>
>>         if (qdf2400_erratum_44_present(&table->header))
>>                 uart = "qdf2400_e44";
>> +       if (xgene_8250_erratum_present(table))
>> +               iotype = "mmio32";
>>
>>         snprintf(opts, sizeof(opts), "%s,%s,0x%llx,%d", uart, iotype,
>>                  table->serial_port.address, baud_rate);
>>
>
> I didn't see a follow up email on this. What was the conclusion to
> this patch series?

I have not received an ack, nack, or gtfo from any of the maintainers
of this file.  Per
./scripts/get_maintainer.pl drivers/acpi/spcr.c
"Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx> (supporter:ACPI)
Len Brown <lenb@xxxxxxxxxx> (supporter:ACPI)
linux-acpi@xxxxxxxxxxxxxxx (open list:ACPI)
linux-kernel@xxxxxxxxxxxxxxx (open list)

Is there someone else I should be directing this patch through?

Thanks,
Jon

> -Loc