Re: [PATCH 05/26] cxl/core: Simplify cxl_dpa_set_mode()

Dan Williams
2024-05-04T04:13:26Z

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;