Re: [RFC PATCH 7/9] cxl/mem: Implement polled mode mailbox

[Jonathan Cameron]

On Tue, 17 Nov 2020 08:34:38 -0800
Ben Widawsky <ben.widawsky@xxxxxxxxx> wrote:

> On 20-11-17 15:31:22, Jonathan Cameron wrote:
> > On Tue, 10 Nov 2020 21:43:54 -0800
> > Ben Widawsky <ben.widawsky@xxxxxxxxx> wrote:
> >   
> > > Create a function to handle sending a command, optionally with a
> > > payload, to the memory device, polling on a result, and then optionally
> > > copying out the payload. The algorithm for doing this come straight out
> > > of the CXL 2.0 specification.
> > > 
> > > Primary mailboxes are capable of generating an interrupt when submitting
> > > a command in the background. That implementation is saved for a later
> > > time.
> > > 
> > > Secondary mailboxes aren't implemented at this time.
> > > 
> > > WARNING: This is untested with actual timeouts occurring.
> > > 
> > > Signed-off-by: Ben Widawsky <ben.widawsky@xxxxxxxxx>  
> > 
> > Question inline for why the preempt / local timer dance is worth bothering with.
> > What am I missing?
> > 
> > Thanks,
> > 
> > Jonathan
> >   
> > > ---
> > >  drivers/cxl/cxl.h |  16 +++++++
> > >  drivers/cxl/mem.c | 107 ++++++++++++++++++++++++++++++++++++++++++++++
> > >  2 files changed, 123 insertions(+)
> > > 
> > > diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
> > > index 482fc9cdc890..f49ab80f68bd 100644
> > > --- a/drivers/cxl/cxl.h
> > > +++ b/drivers/cxl/cxl.h
> > > @@ -21,8 +21,12 @@
> > >  #define CXLDEV_MB_CTRL 0x04
> > >  #define   CXLDEV_MB_CTRL_DOORBELL BIT(0)
> > >  #define CXLDEV_MB_CMD 0x08
> > > +#define   CXLDEV_MB_CMD_PAYLOAD_LENGTH_SHIFT 16
> > >  #define CXLDEV_MB_STATUS 0x10
> > > +#define   CXLDEV_MB_STATUS_RET_CODE_SHIFT 32
> > > +#define   CXLDEV_MB_STATUS_RET_CODE_MASK 0xffff
> > >  #define CXLDEV_MB_BG_CMD_STATUS 0x18
> > > +#define CXLDEV_MB_PAYLOAD 0x20
> > >  
> > >  /* Memory Device */
> > >  #define CXLMDEV_STATUS 0
> > > @@ -114,4 +118,16 @@ static inline u64 __cxl_raw_read_reg64(struct cxl_mem *cxlm, u32 reg)
> > >  
> > >  	return readq(reg_addr + reg);
> > >  }
> > > +
> > > +static inline void cxl_mbox_payload_fill(struct cxl_mem *cxlm, u8 *input,
> > > +					    unsigned int length)
> > > +{
> > > +	memcpy_toio(cxlm->mbox.regs + CXLDEV_MB_PAYLOAD, input, length);
> > > +}
> > > +
> > > +static inline void cxl_mbox_payload_drain(struct cxl_mem *cxlm,
> > > +					     u8 *output, unsigned int length)
> > > +{
> > > +	memcpy_fromio(output, cxlm->mbox.regs + CXLDEV_MB_PAYLOAD, length);
> > > +}
> > >  #endif /* __CXL_H__ */
> > > diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> > > index 9fd2d1daa534..08913360d500 100644
> > > --- a/drivers/cxl/mem.c
> > > +++ b/drivers/cxl/mem.c
> > > @@ -1,5 +1,6 @@
> > >  // SPDX-License-Identifier: GPL-2.0-only
> > >  // Copyright(c) 2020 Intel Corporation. All rights reserved.
> > > +#include <linux/sched/clock.h>
> > >  #include <linux/module.h>
> > >  #include <linux/pci.h>
> > >  #include <linux/io.h>
> > > @@ -7,6 +8,112 @@
> > >  #include "pci.h"
> > >  #include "cxl.h"
> > >  
> > > +struct mbox_cmd {
> > > +	u16 cmd;
> > > +	u8 *payload;
> > > +	size_t payload_size;
> > > +	u16 return_code;
> > > +};
> > > +
> > > +static int cxldev_wait_for_doorbell(struct cxl_mem *cxlm)
> > > +{
> > > +	u64 start, now;
> > > +	int cpu, ret, timeout = 2000000000;
> > > +
> > > +	start = local_clock();
> > > +	preempt_disable();
> > > +	cpu = smp_processor_id();
> > > +	for (;;) {
> > > +		now = local_clock();
> > > +		preempt_enable();  
> > 
> > What do we ever do with this mailbox that is particularly
> > performance critical? I'd like to understand why we care enough
> > to mess around with the preemption changes and local clock etc.
> >   
> 
> It is quite obviously a premature optimization at this point (since we only
> support a single command in QEMU). However, the polling can be anywhere from
> instant to 2 seconds. QEMU implementation aside again, some devices may never
> support interrupts on completion, and so I thought providing a poll function now
> that is capable of working for most [all?] cases was wise.

Definitely seems premature.  I'd want to see real numbers on hardware
to justify this sort of complexity.  Maybe others disagree though.


Jonathan

> 
> > > +		if ((cxl_read_mbox_reg32(cxlm, CXLDEV_MB_CTRL) &
> > > +		     CXLDEV_MB_CTRL_DOORBELL) == 0) {
> > > +			ret = 0;
> > > +			break;
> > > +		}
> > > +
> > > +		if (now - start >= timeout) {
> > > +			ret = -ETIMEDOUT;
> > > +			break;
> > > +		}
> > > +
> > > +		cpu_relax();
> > > +		preempt_disable();
> > > +		if (unlikely(cpu != smp_processor_id())) {
> > > +			timeout -= (now - start);
> > > +			cpu = smp_processor_id();
> > > +			start = local_clock();
> > > +		}
> > > +	}
> > > +
> > > +	return ret;
> > > +}
> > > +
> > > +/*
> > > + * Returns 0 if the doorbell transaction was successful from a protocol level.
> > > + * Caller should check the return code in @mbox_cmd to make sure it succeeded.
> > > + */
> > > +static int __maybe_unused cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm, struct mbox_cmd *mbox_cmd)
> > > +{
> > > +	u64 cmd, status;
> > > +	int rc;
> > > +
> > > +	lockdep_assert_held(&cxlm->mbox_lock);
> > > +
> > > +	/*
> > > +	 * Here are the steps from 8.2.8.4 of the CXL 2.0 spec.
> > > +	 *   1. Caller reads MB Control Register to verify doorbell is clear
> > > +	 *   2. Caller writes Command Register
> > > +	 *   3. Caller writes Command Payload Registers if input payload is non-empty
> > > +	 *   4. Caller writes MB Control Register to set doorbell
> > > +	 *   5. Caller either polls for doorbell to be clear or waits for interrupt if configured
> > > +	 *   6. Caller reads MB Status Register to fetch Return code
> > > +	 *   7. If command successful, Caller reads Command Register to get Payload Length
> > > +	 *   8. If output payload is non-empty, host reads Command Payload Registers
> > > +	 */
> > > +
> > > +	cmd = mbox_cmd->cmd;
> > > +	if (mbox_cmd->payload_size) {
> > > +		/* #3 */  
> > 
> > Having just given the steps above, having them out of order feels like it needs
> > a comment to state why.
> >   
> > > +		cmd |= mbox_cmd->payload_size
> > > +		       << CXLDEV_MB_CMD_PAYLOAD_LENGTH_SHIFT;
> > > +		cxl_mbox_payload_fill(cxlm, mbox_cmd->payload, mbox_cmd->payload_size);
> > > +	}
> > > +
> > > +	/* #2 */
> > > +	cxl_write_mbox_reg64(cxlm, CXLDEV_MB_CMD, cmd);
> > > +
> > > +	/* #4 */
> > > +	cxl_write_mbox_reg32(cxlm, CXLDEV_MB_CTRL, CXLDEV_MB_CTRL_DOORBELL);
> > > +
> > > +	/* #5 */
> > > +	rc = cxldev_wait_for_doorbell(cxlm);
> > > +	if (rc == -ETIMEDOUT) {
> > > +		dev_warn(&cxlm->pdev->dev, "Mailbox command timed out\n");
> > > +		return rc;
> > > +	}
> > > +
> > > +	/* #6 */
> > > +	status = cxl_read_mbox_reg64(cxlm, CXLDEV_MB_STATUS);
> > > +	cmd = cxl_read_mbox_reg64(cxlm, CXLDEV_MB_CMD);
> > > +
> > > +	mbox_cmd->return_code = (status >> CXLDEV_MB_STATUS_RET_CODE_SHIFT) &
> > > +				CXLDEV_MB_STATUS_RET_CODE_MASK;
> > > +
> > > +	/* There was a problem, let the caller deal with it */
> > > +	if (mbox_cmd->return_code != 0)
> > > +		return 0;
> > > +
> > > +	/* #7 */
> > > +	mbox_cmd->payload_size = cmd >> CXLDEV_MB_CMD_PAYLOAD_LENGTH_SHIFT;
> > > +
> > > +	/* #8 */
> > > +	if (mbox_cmd->payload_size)
> > > +		cxl_mbox_payload_drain(cxlm, mbox_cmd->payload, mbox_cmd->payload_size);
> > > +
> > > +	return 0;
> > > +}
> > > +
> > >  static int cxl_mem_mbox_get(struct cxl_mem *cxlm)
> > >  {
> > >  	u64 md_status;  
> >