Re: [PATCH v9 03/27] cxl: add capabilities field to cxl_dev_state and cxl_port

On Fri, 3 Jan 2025 07:20:48 +0000
Alejandro Lucero Palau [off-list ref] wrote:

On 1/2/25 14:36, Jonathan Cameron wrote:

quoted

On Mon, 30 Dec 2024 21:44:21 +0000
[off-list ref] wrote:
 

quoted

From: Alejandro Lucero <redacted>

Type2 devices have some Type3 functionalities as optional like an mbox
or an hdm decoder, and CXL core needs a way to know what an CXL accelerator
implements.

Add a new field to cxl_dev_state for keeping device capabilities as
discovered during initialization. Add same field to cxl_port as registers
discovery is also used during port initialization.

Signed-off-by: Alejandro Lucero <redacted>
Reviewed-by: Ben Cheatham <redacted>
Reviewed-by: Fan Ni <redacted>  

Comment in thread on v8.  I don't see a reason to have any specific
bitmap length - just use a final entry in the enum without a value set
to let us know how long it actually is.  

I could do this but it implies to clear/zeroing the bitmaps with the 
final entry value and to mask bitmaps with that when comparing them.

Yes but that is automatic if you use the bitmap functions throughout.

I tried to avoid the masking, and it led to that use of sizeof and then 
CXL_MAX_CAPS=64.

Don't avoid it. You are creating maintenance pain for a bit of unnecessary
micro optimization.  Just make sure to treat this bitmap as a bitmap
in all paths and there will be not reason for a reviewer to ever have
to care what this value is and whether enough bits are zero etc.

Jonathan

quoted

Using the bit / bitmap functions should work fine without constraining
that to any particular value - also allowing for greater than 64 entries
with no need to fix up call sites etc.

 

quoted

diff --git a/drivers/cxl/core/regs.c b/drivers/cxl/core/regs.c
index 59cb35b40c7e..144ae9eb6253 100644
--- a/drivers/cxl/core/regs.c
+++ b/drivers/cxl/core/regs.c

@@ -4,6 +4,7 @@
+enum cxl_dev_cap {
+	/* capabilities from Component Registers */
+	CXL_DEV_CAP_RAS,
+	CXL_DEV_CAP_HDM,
+	/* capabilities from Device Registers */
+	CXL_DEV_CAP_DEV_STATUS,
+	CXL_DEV_CAP_MAILBOX_PRIMARY,
+	CXL_DEV_CAP_MEMDEV,
+	CXL_MAX_CAPS = 64  

As in v8. I'm not seeing any reason for this.  If you need
a bitmap to be a particular number of unsigned longs, then that
code should be fixed. (only exception being compile time constant
bitmaps where this is tricky to do!)

Obviously I replied with that to v8 after you posted this
so time machines aside no way you could have acted on it yet.


Jonathan
 

quoted

+};
+
  struct cxl_dev_state;
  struct device;