linux-kernel - Re: [PATCH v3] dcdbas: Add support for WSMT ACPI table

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Date:   Thu, 14 Jun 2018 17:31:08 -0500
From:   Stuart Hayes <stuart.w.hayes@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Darren Hart <dvhart@...radead.org>, Douglas_Warzecha@...l.com,
        Mario Limonciello <mario.limonciello@...l.com>,
        Jared.Dominguez@...l.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Platform Driver <platform-driver-x86@...r.kernel.org>
Subject: Re: [PATCH v3] dcdbas: Add support for WSMT ACPI table

On 6/14/2018 12:25 PM, Andy Shevchenko wrote:
> On Thu, Jun 14, 2018 at 5:22 PM, Stuart Hayes <stuart.w.hayes@...il.com> wrote:
>> On 6/13/2018 3:54 AM, Andy Shevchenko wrote:
> 
>>>> +                * Provide physical address of command buffer field within
>>>> +                * the struct smi_cmd... can't use virt_to_phys on smi_cmd
>>>> +                * because address may be from memremap.
>>>
>>> Wait, memremap() might return a virtual address. How we be sure that
>>> we got still physical address here?
> 
>> Before this patch, the address in smi_cmd always came from an alloc, so
>> virt_to_phys() was used to get the physical address here.  With WSMT, we
>> could be using a BIOS-provided buffer for SMI, in which case the address in
>> smi_cmd will come from memremap(), so we can't use virt_to_phys() on it.
>> So instead I changed this to use the physical address of smi_data_buf that
>> is stored in smi_data_buf_phys_addr, which will be valid regardless of how
>> the address of smi_data_buf was generated.
> 
> Yes, but what does guarantee that memremap() will return you still
> physical address?
> 

Sorry, I'm not sure I understand the question.

Up to now, this driver always just allocated a buffer from main memory that
it used to send/receive information from BIOS when it generated a SMI.  That's
what smi_cmd points to where this comment is.  And it was safe to use
virt_to_phys() on this address.

With this patch, though, the driver may now be using a buffer that isn't part
of main memory--it could now be using a buffer that BIOS provided the physical
address for, and this would not be part of main memory.  So smi_cmd may contain
a virtual address that memremap() provided.  And because memremap() is just
like ioremap(), the driver can no longer use virt_to_phys(smi_cmd) to get the
physical address of the buffer.

My comment is just pointing that out... I was trying to say, "the code can't
use virt_to_phys(smi_cmd) to get the virtual address here".

memremap() should always return a virtual address that points to the physical
address we send it (unless it fails of course).

>>>> +               return 0;
>>>> +
>>>> +       /* Scan for EPS (entry point structure) */
>>>> +       for (addr = (u8 *)__va(0xf0000);
>>>> +            addr < (u8 *)__va(0x100000 - sizeof(struct smm_eps_table));
>>>
>>>> +            addr += 1) {
>>>
>>> This wasn't commented IIRC and changed. So, why?
> 
>> I changed this is response to your earlier comment (7 june)... you had pointed
>> out that it would be better if I put an "if (eps) break;" inside the for loop
>> instead of having "&& !eps" in the condition of the for loop.  I put the note
>> "Changed loop searching 0xf0000 to be more readable" in the list of changes for
>> patch version v3 to cover this change.
> 
> Thanks, but here I meant += 1 vs += 16 step.
> 

Sorry, I thought I had answered this earlier.  The spec does not say that the EPS
table will be on a 16-byte boundary.  And I just added a printk in this driver to
see where it is on the system I had at hand, and it isn't on a 16-byte boundary:

[ 4680.192542] dcdbas - EPS table at 000000005761efb7
[ 4680.194012] dcdbas dcdbas: WSMT found, using firmware-provided SMI buffer.
[ 4680.195327] dcdbas dcdbas: Dell Systems Management Base Driver (version 5.6.0-3.3)