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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Thu, 28 Jun 2018 02:52:06 +0300
Message-ID: <CAHp75VfL3v8U6o36-e3+XPL53sb9wMUOXsL6gn3XE4=-kJjuKQ@mail.gmail.com>
Subject: Re: [PATCH v3] dcdbas: Add support for WSMT ACPI table
To:     Stuart Hayes <stuart.w.hayes@gmail.com>
Cc:     Darren Hart <dvhart@infradead.org>,
        Mario Limonciello <mario.limonciello@dell.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Platform Driver <platform-driver-x86@vger.kernel.org>
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On Fri, Jun 15, 2018 at 1:31 AM, Stuart Hayes <stuart.w.hayes@gmail.com> wrote:
> On 6/14/2018 12:25 PM, Andy Shevchenko wrote:
>> On Thu, Jun 14, 2018 at 5:22 PM, Stuart Hayes <stuart.w.hayes@gmail.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.

Hmm... But is it CPU address or bus address what BIOS provides?

If it's a CPU address why do you need to call memremap() on it in the
first place?
I could guess that you want to access it from CPU side and rather
would get faults.

>  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.

Yes, and ioremap() is dedicated for the resources that are not
available directly by the memory accesses, but rather require some bus
transactions (like MMIO).

>
> 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".
>

Please, add bits from above paragraphs to elaborate this in the comment.

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

>>>>> +       /* 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:

Oh, that's sad.

Btw, does XSDT have a link to this table?

> [ 4680.192542] dcdbas - EPS table at 000000005761efb7

Can you, by the way, dump some bytes around this address, using
print_hex_dump_bytes();

where the adrress is aligned to let say 32 byte boundary and size like 64 bytes?

> [ 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)

OK, now the most important question, did you investigate "SMM
Communication ACPI Table"?
Can you utilize information in it?

-- 
With Best Regards,
Andy Shevchenko