linux-kernel - Re: [PATCH 05/26] cxl/core: Simplify cxl_dpa_set

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Date: Fri, 3 May 2024 21:13:20 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Ira Weiny <ira.weiny@...el.com>, Alison Schofield
	<alison.schofield@...el.com>
CC: Dave Jiang <dave.jiang@...el.com>, Fan Ni <fan.ni@...sung.com>, "Jonathan
 Cameron" <Jonathan.Cameron@...wei.com>, Navneet Singh
	<navneet.singh@...el.com>, Dan Williams <dan.j.williams@...el.com>,
	"Davidlohr Bueso" <dave@...olabs.net>, Vishal Verma
	<vishal.l.verma@...el.com>, <linux-btrfs@...r.kernel.org>,
	<linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/26] cxl/core: Simplify cxl_dpa_set_mode()

Ira Weiny wrote:
> Alison Schofield wrote:
> > On Sun, Mar 24, 2024 at 04:18:08PM -0700, Ira Weiny wrote:
> > > cxl_dpa_set_mode() checks the mode for validity two times, once outside
> > > of the DPA RW semaphore and again within.
> > 
> > Not true.
> 
> Sorry for not being clear.  It does check the mode 2x but not for
> validity.  I'll clarify.
> 
> > It only checks mode once before the lock. It checks for
> > capacity after the lock. If it didn't check mode before the lock,
> > then unsupported modes would fall through.
> 
> But we can check the mode 1 time and either check the size or fail.
> 
> > 
> > > The function is not in a critical path.
> > 
> > Implying what here?  OK to check twice (even though it wasn't)
> > or OK to expand scope of locking.
> 
> Implying that checking the mode outside the lock is not required.
> 
> > 
> > > Prior to Dynamic Capacity the extra check was not much
> > > of an issue.  The addition of DC modes increases the complexity of
> > > the check.
> > > 
> > > Simplify the mode check before adding the more complex DC modes.
> > > 
> > 
> > The addition of the DC mode check doesn't seem complex.
> 
> It is if you have to check it 2 times.
> 
> > 
> > Pardon my picking at the words, but if you'd like to refactor the
> > function, just say so. The final result is a bit more readable, but
> > also adding the DC mode checks without refactoring would read fine
> > also.
> 
> When I added the DC mode to this function without this refactoring it was
> quite a bit more code and ugly IMO.  So this cleanup helped.  If I were
> not adding the DC code there would be much less reason to change this
> function.

Where did the "quite a bit more code" come from? A change that moves
unnecessary code under a lock and is larger than just incrementally
extending the status quo does not feel like a cleanup.

diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
index 7d97790b893d..0dc886bc22c6 100644
--- a/drivers/cxl/core/hdm.c
+++ b/drivers/cxl/core/hdm.c
@@ -411,11 +411,12 @@ int cxl_dpa_set_mode(struct cxl_endpoint_decoder *cxled,
        struct cxl_memdev *cxlmd = cxled_to_memdev(cxled);
        struct cxl_dev_state *cxlds = cxlmd->cxlds;
        struct device *dev = &cxled->cxld.dev;
-       int rc;
+       int rc, dcd;
 
        switch (mode) {
        case CXL_DECODER_RAM:
        case CXL_DECODER_PMEM:
+       case CXL_DECODER_DC0 ... CXL_DECODER_DC7:
                break;
        default:
                dev_dbg(dev, "unsupported mode: %d\n", mode);
@@ -442,6 +443,11 @@ int cxl_dpa_set_mode(struct cxl_endpoint_decoder *cxled,
                rc = -ENXIO;
                goto out;
        }
+       dcd = dc_mode_to_region_index(mode);
+       if (resource_size(&cxlds->dc_res[dcd]) == 0) {
+               dev_dbg(dev, "no available dynamic capacity\n");
+               goto out;
+       }
 
        cxled->mode = mode;
        rc = 0;