linux-kernel - Re: [PATCH v12] firmware: google: Implement cbmem in sysfs driver

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Message-ID: <Yzxl2oczTtwEvIqt@chromium.org>
Date:   Tue, 4 Oct 2022 10:56:58 -0600
From:   Jack Rosenthal <jrosenth@...omium.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, chrome-platform@...ts.linux.dev,
        Stephen Boyd <swboyd@...omium.org>,
        Tzung-Bi Shih <tzungbi@...nel.org>,
        Guenter Roeck <groeck@...omium.org>,
        Julius Werner <jwerner@...omium.org>
Subject: Re: [PATCH v12] firmware: google: Implement cbmem in sysfs driver

On 2022-10-04 at 10:51 +0200, Greg KH wrote:
> > +		A list of ids known to Coreboot can be found in the coreboot
> > +		source tree at
> > +		``src/commonlib/bsd/include/commonlib/bsd/cbmem_id.h``.
> 
> That will not age well, why not point to the reference in the kernel
> tree instead?

There is no copy in the kernel tree.

> > +What:		/sys/bus/coreboot/devices/cbmem-<id>/address
> > +Date:		August 2022
> > +Contact:	Jack Rosenthal <jrosenth@...omium.org>
> > +Description:
> > +		This is the pyhsical memory address that the CBMEM entry's data
> > +		begins at.
> 
> In hex?  Decimal?
> 
> > +
> > +What:		/sys/bus/coreboot/devices/cbmem-<id>/size
> > +Date:		August 2022
> > +Contact:	Jack Rosenthal <jrosenth@...omium.org>
> > +Description:
> > +		This is the size of the CBMEM entry's data.
> 
> In hex?  Decimal?  Octal?  Binary?  Be specific please :)

Added "hexadecimal" and an example for both in v13.

> > +What:		/sys/bus/coreboot/devices/cbmem-<id>/id
> > +Date:		August 2022
> > +Contact:	Jack Rosenthal <jrosenth@...omium.org>
> > +Description:
> > +		This is the CBMEM id corresponding to the entry.
> 
> so "id" is the same as "<id>" here?  Why is that needed?

Removed in v13, agreee it's reduntant with the device name now.

> > +	  Say Y here to enable the kernel to search for Coreboot CBMEM
> > +	  entries, and expose the memory for each entry in sysfs under
> > +	  /sys/bus/coreboot/devices/cbmem-<id>.
> 
> Module name?

Added in v13.

> 
> > +
> >  config GOOGLE_COREBOOT_TABLE
> >  	tristate "Coreboot Table Access"
> >  	depends on HAS_IOMEM && (ACPI || OF)
> > diff --git a/drivers/firmware/google/Makefile b/drivers/firmware/google/Makefile
> > index d17caded5d88..8151e323cc43 100644
> > --- a/drivers/firmware/google/Makefile
> > +++ b/drivers/firmware/google/Makefile
> > @@ -7,5 +7,8 @@ obj-$(CONFIG_GOOGLE_MEMCONSOLE)            += memconsole.o
> >  obj-$(CONFIG_GOOGLE_MEMCONSOLE_COREBOOT)   += memconsole-coreboot.o
> >  obj-$(CONFIG_GOOGLE_MEMCONSOLE_X86_LEGACY) += memconsole-x86-legacy.o
> >  
> > +# Must come after coreboot_table.o, as this driver depends on that bus type.
> 
> Doesn't the linker handle this for us?

Not in the case of compiling as a built-in module: I observed in this
scenario the order in the Makefile deterimined the module initialization
order, and, if this were to be listed alphabetically, the coreboot_table
module would not have been loaded before the cbmem module.

> > +	entry->size = dev->cbmem_entry.entry_size;
> 
> Ah nevermind you set the size here.

The size that stat reports is still 0, as when creating this as a device
attribute, the size is not known until the driver is probed.  I observed
this in some other sysfs attributes, so I imagine it's a common pattern.

> > +/* Corresponds to LB_TAG_CBMEM_ENTRY */
> > +struct lb_cbmem_entry {
> > +	u32 tag;
> > +	u32 size;
> 
> little or big endian?

It's the native host endianness, as coreboot wrote these tables from the
same CPU that Linux is running on.