netdev - Re: [PATCH v9 14/27] sfc: create type2 cxl memdev

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Message-ID: <67902c3a2da7d_20fa294a4@dwillia2-xfh.jf.intel.com.notmuch>
Date: Tue, 21 Jan 2025 15:22:34 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Alejandro Lucero Palau <alucerop@....com>, Dan Williams
	<dan.j.williams@...el.com>, <alejandro.lucero-palau@....com>,
	<linux-cxl@...r.kernel.org>, <netdev@...r.kernel.org>, <edward.cree@....com>,
	<davem@...emloft.net>, <kuba@...nel.org>, <pabeni@...hat.com>,
	<edumazet@...gle.com>, <dave.jiang@...el.com>
Subject: Re: [PATCH v9 14/27] sfc: create type2 cxl memdev

Alejandro Lucero Palau wrote:
> 
> On 1/18/25 02:41, Dan Williams wrote:
> > alejandro.lucero-palau@ wrote:
> >> From: Alejandro Lucero <alucerop@....com>
> >>
> >> Use cxl API for creating a cxl memory device using the type2
> >> cxl_dev_state struct.
> >>
> >> Signed-off-by: Alejandro Lucero <alucerop@....com>
> >> Reviewed-by: Martin Habets <habetsm.xilinx@...il.com>
> >> Reviewed-by: Fan Ni <fan.ni@...sung.com>
> >> Acked-by: Edward Cree <ecree.xilinx@...il.com>
> >> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> >> ---
> >>   drivers/net/ethernet/sfc/efx_cxl.c | 9 +++++++++
> >>   1 file changed, 9 insertions(+)
> >>
> >> diff --git a/drivers/net/ethernet/sfc/efx_cxl.c b/drivers/net/ethernet/sfc/efx_cxl.c
> >> index 911f29b91bd3..f4bf137fd878 100644
> >> --- a/drivers/net/ethernet/sfc/efx_cxl.c
> >> +++ b/drivers/net/ethernet/sfc/efx_cxl.c
> >> @@ -96,10 +96,19 @@ int efx_cxl_init(struct efx_probe_data *probe_data)
> >>   	 */
> >>   	cxl_set_media_ready(cxl->cxlds);
> > It is unfortunate the media_ready is just being used as an
> > offline/online flag for memdevs. I would be open to just switching to
> > typical device online/offline semantics and drop the "media_ready" flag.
> 
> 
> Not sure I understand the semantics here. Note our device, as the 
> related register being optional, has no way for asking the HW about this 
> (by CXL means), mainly because it is not necessary. I guess this 
> hardware state is there because it can be needed, but again, not sure I 
> understand media-ready vs memory online.

The ->media_ready flag gates capacity discovery for memory expanders
(no impact to accelerators) *and* it gates whether cxl_mem_probe() even
attempts to talk to the device.

The concern is that it should be removed from 'struct cxl_dev_state' if
it cannot have common meaning across all CXL.mem capable devices. One
way to remove it from 'struct cxl_dev_state' while still preserving the
memory expander use case is to notice that ->media_ready is effectively
identical to the 'struct device' @offline flag.

For now, just have the accelerator driver set ->media_ready and we can
work on cleaning up that direct 'struct cxl_dev_state' touch later.

> >> +	cxl->cxlmd = devm_cxl_add_memdev(&pci_dev->dev, cxl->cxlds);
> >> +	if (IS_ERR(cxl->cxlmd)) {
> >> +		pci_err(pci_dev, "CXL accel memdev creation failed");
> >> +		rc = PTR_ERR(cxl->cxlmd);
> >> +		goto err_memdev;
> >> +	}
> >> +
> >>   	probe_data->cxl = cxl;
> >>   
> >>   	return 0;
> >>   
> >> +err_memdev:
> >> +	cxl_release_resource(cxl->cxlds, CXL_RES_RAM);
> >>   err_resource_set:
> >>   	kfree(cxl->cxlds);
> > In general a function should not mix devm and goto as that is a recipe
> > for bugs.
> >
> > The bug I see here is that devm_cxl_add_memdev() runs the teardown flow
> > *after* efx_pci_probe() returns an error code. That happens in
> > device_unbind_cleanup(), but when it goes to cleanup endpoint decoders
> > and anything else that might reference @cxlds it crashes because @cxlds
> > is long gone.
> >
> > So if you use devm_cxl_add_memdev() then cxlds must be devm allocated as
> > well to make sure it gets freed in the proper reverse order.
> 
> That is true and I think it is easy to fix, even with your changes for 
> patch 1.

Yes, just add a devm_cxl_del_memdev() export for now that can be used
unwind CXL core objects without requiring the driver to switch to devm
more broadly.