netdev - Re: [PATCH v19 22/22] sfc: support pio mapping based on cxl

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Message-ID: <20251007154845.00001afa@huawei.com>
Date: Tue, 7 Oct 2025 15:48:45 +0100
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: <alejandro.lucero-palau@....com>
CC: <linux-cxl@...r.kernel.org>, <netdev@...r.kernel.org>,
	<dan.j.williams@...el.com>, <edward.cree@....com>, <davem@...emloft.net>,
	<kuba@...nel.org>, <pabeni@...hat.com>, <edumazet@...gle.com>,
	<dave.jiang@...el.com>, Alejandro Lucero <alucerop@....com>
Subject: Re: [PATCH v19 22/22] sfc: support pio mapping based on cxl

On Mon, 6 Oct 2025 11:01:30 +0100
<alejandro.lucero-palau@....com> wrote:

> From: Alejandro Lucero <alucerop@....com>
> 
> A PIO buffer is a region of device memory to which the driver can write a
> packet for TX, with the device handling the transmit doorbell without
> requiring a DMA for getting the packet data, which helps reducing latency
> in certain exchanges. With CXL mem protocol this latency can be lowered
> further.
> 
> With a device supporting CXL and successfully initialised, use the cxl
> region to map the memory range and use this mapping for PIO buffers.
> 
> Add the disabling of those CXL-based PIO buffers if the callback for
> potential cxl endpoint removal by the CXL code happens.
> 
> Signed-off-by: Alejandro Lucero <alucerop@....com>

A few minor things inline.  The ifdef complexity in here is moderately
nasty though.  Might be worth seeing if any of that can be moved
to stubs or IS_ENABLED() checks.


> diff --git a/drivers/net/ethernet/sfc/efx.h b/drivers/net/ethernet/sfc/efx.h
> index 45e191686625..057d30090894 100644
> --- a/drivers/net/ethernet/sfc/efx.h
> +++ b/drivers/net/ethernet/sfc/efx.h
> @@ -236,5 +236,4 @@ static inline bool efx_rwsem_assert_write_locked(struct rw_semaphore *sem)
>  
>  int efx_xdp_tx_buffers(struct efx_nic *efx, int n, struct xdp_frame **xdpfs,
>  		       bool flush);
> -

stray change that you should clean out.

>  #endif /* EFX_EFX_H */
> diff --git a/drivers/net/ethernet/sfc/efx_cxl.c b/drivers/net/ethernet/sfc/efx_cxl.c
> index 79fe99d83f9f..a84ce45398c1 100644
> --- a/drivers/net/ethernet/sfc/efx_cxl.c
> +++ b/drivers/net/ethernet/sfc/efx_cxl.c
> @@ -11,6 +11,7 @@
>  #include <cxl/pci.h>
>  #include "net_driver.h"
>  #include "efx_cxl.h"
> +#include "efx.h"
>  
>  #define EFX_CTPIO_BUFFER_SIZE	SZ_256M
>  
> @@ -20,6 +21,7 @@ int efx_cxl_init(struct efx_probe_data *probe_data)
>  	struct pci_dev *pci_dev = efx->pci_dev;
>  	resource_size_t max_size;
>  	struct efx_cxl *cxl;
> +	struct range range;
>  	u16 dvsec;
>  	int rc;
>  
> @@ -119,19 +121,40 @@ int efx_cxl_init(struct efx_probe_data *probe_data)
>  	cxl->efx_region = cxl_create_region(cxl->cxlrd, &cxl->cxled, 1);
>  	if (IS_ERR(cxl->efx_region)) {
>  		pci_err(pci_dev, "CXL accel create region failed");
> -		cxl_put_root_decoder(cxl->cxlrd);
> -		cxl_dpa_free(cxl->cxled);
> -		return PTR_ERR(cxl->efx_region);
> +		rc = PTR_ERR(cxl->efx_region);
> +		goto err_dpa;

It's a somewhat trivial thing but you could reduce churn by
moving the err_dpa block introduction back to where this lot
was first added. 

> +	}
> +
> +	rc = cxl_get_region_range(cxl->efx_region, &range);
> +	if (rc) {
> +		pci_err(pci_dev, "CXL getting regions params failed");
> +		goto err_detach;
> +	}
> +
> +	cxl->ctpio_cxl = ioremap(range.start, range.end - range.start + 1);
> +	if (!cxl->ctpio_cxl) {
> +		pci_err(pci_dev, "CXL ioremap region (%pra) failed", &range);
> +		rc = -ENOMEM;
> +		goto err_detach;
>  	}
>  
>  	probe_data->cxl = cxl;
> +	probe_data->cxl_pio_initialised = true;
>  
>  	return 0;
> +
> +err_detach:
> +	cxl_decoder_detach(NULL, cxl->cxled, 0, DETACH_INVALIDATE);
> +err_dpa:
> +	cxl_put_root_decoder(cxl->cxlrd);
> +	cxl_dpa_free(cxl->cxled);
> +	return rc;
>  }
>  
>  void efx_cxl_exit(struct efx_probe_data *probe_data)
>  {
> -	if (probe_data->cxl) {
> +	if (probe_data->cxl_pio_initialised) {
> +		iounmap(probe_data->cxl->ctpio_cxl);
>  		cxl_decoder_detach(NULL, probe_data->cxl->cxled, 0,
>  				   DETACH_INVALIDATE);
>  		cxl_dpa_free(probe_data->cxl->cxled);