linux-kernel - question about ioremap

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Message-ID: <20130811165031.GA4439@thinkpad.fritz.box>
Date:	Sun, 11 Aug 2013 18:50:31 +0200
From:	Andreas Werner <wernerandy@....de>
To:	linux-kernel@...r.kernel.org
Subject: question about ioremap_cache and PAT

Hi i have a question about ioremap_cache and the resulting PAT attribute on X86 system. If I configure the mtrr to Write-Through for an adress range, and call ioremap_cache to map the mmio, the resulting PAT attribute is set to UC.
If I check the Intel document IA-32 SDM vol 3a, the resulting PAT attribute should be WB.

I found the function pat_x_mtrr_type in arch/x86/mm/pat.c where the resulting attribute is returned. There will be always UC return expect if the MTRR is set to WB.

Why is there only WB or UC returned? In the Intel document there are a lot of combinations "allowed".

I need a Attribute of WT, so what i did is to modify the pat_x_mtrr_type function to return also WB if the MTRR is set to WT.   

Is this a solution to solve that or whats the reasion why the kernel doesn´t support this combination?

Best regards

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Best regards
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