linux-kernel - Re: [PATCH v9 01/19] cxl/mbox: Flag support for Dynamic Capacity Devices (DCD)

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Message-ID: <aBpa02RH0CnRv6Jl@lg>
Date: Tue, 6 May 2025 11:54:11 -0700
From: Fan Ni <nifan.cxl@...il.com>
To: Ira Weiny <ira.weiny@...el.com>
Cc: Fan Ni <nifan.cxl@...il.com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Alison Schofield <alison.schofield@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>, linux-cxl@...r.kernel.org,
	nvdimm@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 01/19] cxl/mbox: Flag support for Dynamic Capacity
 Devices (DCD)

On Tue, May 06, 2025 at 11:09:09AM -0500, Ira Weiny wrote:
> Fan Ni wrote:
> > On Mon, Apr 14, 2025 at 03:19:50PM +0100, Jonathan Cameron wrote:
> > > On Sun, 13 Apr 2025 17:52:09 -0500
> > > Ira Weiny <ira.weiny@...el.com> wrote:
> 
> [snip]
> 
> > > 
> > > > +
> > > > +static bool cxl_verify_dcd_cmds(struct cxl_memdev_state *mds, unsigned long *cmds_seen)
> > > 
> > > It's not immediately obvious to me what the right behavior
> > > from something called cxl_verify_dcd_cmds() is.  A comment might help with that.
> > > 
> > > I think all it does right now is check if any bits are set. In my head
> > > it was going to check that all bits needed for a useful implementation were
> > > set. I did have to go check what a 'logical and' of a bitmap was defined as
> > > because that bit of the bitmap_and() return value wasn't obvious to me either!
> > 
> > The code only checks if any DCD command (48xx) is supported, if any is
> > set, it will set "dcd_supported".
> > As you mentioned, it seems we should check all the related commands are
> > supported, otherwise it is not valid implementation.
> > 
> > Fan
> > > 
> > > 
> > > > +{
> > > > +	DECLARE_BITMAP(all_cmds, CXL_DCD_ENABLED_MAX);
> > > > +	DECLARE_BITMAP(dst, CXL_DCD_ENABLED_MAX);
> > > > +
> > > > +	bitmap_fill(all_cmds, CXL_DCD_ENABLED_MAX);
> > > > +	return bitmap_and(dst, cmds_seen, all_cmds, CXL_DCD_ENABLED_MAX);
> 
> Yea... so this should read:
> 
> ...
> 	bitmap_and(dst, cmds_seen, all_cmds, CXL_DCD_ENABLED_MAX);
> 	return bitmap_equal(dst, all_cmds, CXL_DCD_ENABLED_MAX);
Maybe only 
    return bitmap_equal(cmds_seen, all_cmds, CXL_DCD_ENABLED_MAX)?

Fan
> ...
> 
> Of course if a device has set any of these commands true it better have
> set them all.  Otherwise the device is broken and it will fail in bad
> ways.
> 
> But I agree with both of you that this is much better and explicit that
> something went wrong.  A dev_dbg() might be in order to debug such an
> issue.
> 
> Ira
> 
> [snip]

-- 
Fan Ni