linux-kernel - Re: [PATCH 5/6] x86/microcode/intel_staging: Support mailbox data transfer

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Message-ID: <99fe30bc-47cf-4910-bd80-3eb743ef0548@intel.com>
Date: Thu, 20 Mar 2025 16:42:37 -0700
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, <linux-kernel@...r.kernel.org>
CC: <x86@...nel.org>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<bp@...en8.de>, <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 5/6] x86/microcode/intel_staging: Support mailbox data
 transfer

On 2/18/2025 12:54 PM, Dave Hansen wrote:
> 
> BTW, is this kind of read/write normal for MMIO reads? It looks really
> goofy to me, but I don't deal with devices much.

Yeah, looking at some device code, functions like memcpy_toio() and 
__iowrite*_copy() seem to assume sufficient MMIO space to copy data from 
kernel memory. I also noticed that some other mailbox code calls back 
one of these functions.

So, I think your assessment is correct:

   > To me, it's much more akin to talking over a serial port than how I
   > think of devices attached via MMIO.

Certainly, this staging mailbox looks somewhat similar to a serial port. 
I suspect the staging portion is quite resource-limited. That appears to 
be one of key characteristics of this interface, which I should 
highlight more comprehensively.

Thanks,
Chang