linux-kernel - Re: [PATCH v3 1/2] firmware: google: Expose CBMEM over sysfs

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Message-ID: <5e28913c.1c69fb81.8a690.7308@mx.google.com>
Date:   Wed, 22 Jan 2020 10:15:23 -0800
From:   Stephen Boyd <swboyd@...omium.org>
To:     Julius Werner <jwerner@...omium.org>, patrick.rudolph@...ements.com
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Allison Randal <allison@...utok.net>,
        Alexios Zavras <alexios.zavras@...el.com>,
        Samuel Holland <samuel@...lland.org>
Subject: Re: [PATCH v3 1/2] firmware: google: Expose CBMEM over sysfs

Quoting patrick.rudolph@...ements.com (2019-12-19 23:12:22)
> On Mon, 2019-12-09 at 22:54 -0800, Julius Werner wrote:
> > > +static int cbmem_probe(struct coreboot_device *cdev)
> > > +{
> > > +       struct device *dev = &cdev->dev;
> > > +       struct cb_priv *priv;
> > > +       int err;
> > > +
> > > +       priv = kzalloc(sizeof(*priv), GFP_KERNEL);
> > > +       if (!priv)
> > > +               return -ENOMEM;
> > > +
> > > +       memcpy(&priv->entry, &cdev->cbmem_entry, sizeof(priv-
> > > >entry));
> > > +
> > > +       priv->remap = memremap(priv->entry.address,
> > > +                              priv->entry.entry_size,
> > > MEMREMAP_WB);
> > 
> > We've just been discussing some problems with CBMEM areas and memory
> > mapping types in Chrome OS. CBMEM is not guaranteed to be page-
> > aligned
> > (at least not the "small" entries), but the kernel can only assign
> > memory attributes for a page at a time (and refuses to map the same
> > area twice with two different memory types, for good reason). So if
> > CBMEM entries sharing a page are mapped as writeback by one driver
> > but
> > uncached by the other, things break.
> > 
> > There are some CBMEM entries that need to be mapped uncached (e.g.
> > the
> > ACPI UCSI table, which isn't even handled by anything using this
> > CBMEM
> > code) and others for which it would make more sense (e.g. the memory
> > console, where firmware may add more lines at runtime), but I don't
> > think there are any regions that really *need* to be writeback. None
> > of the stuff accessing these areas should access them often enough
> > that caching matters, and I think it's generally more common to map
> > firmware memory areas as uncached anyway. So how about we standardize
> > on mapping it all uncached to avoid any attribute clashes? (That
> > would
> > mean changing the existing VPD and memconsole drivers to use
> > ioremap(), too.)
> 
> I wasn't aware that CBMEM is used for DMA as there's no such concept in
> coreboot yet. For me it looks like the UCSI is regular DRAM mapped as
> WB accessed by the ACPI interpreter.
> I'll prepare a new patch-set using ioremap in all drivers that access
> CBMEM.
> 

We shouldn't use ioremap() here as this isn't I/O memory. It's just
regular memory that wants be mapped with some particular set of
attributes, hence the use of memremap().