Re: [PATCH v2 2/8] cxl/mem: Find device capabilities

Ben Widawsky
2021-02-10T18:16:16Z

On 21-02-10 08:55:57, Ben Widawsky wrote:
> On 21-02-10 15:07:59, Jonathan Cameron wrote:
> > On Wed, 10 Feb 2021 13:32:52 +0000
> > Jonathan Cameron <Jonathan.Cameron@Huawei.com> wrote:
> >
> > > On Tue, 9 Feb 2021 16:02:53 -0800
> > > Ben Widawsky <ben.widawsky@intel.com> wrote:
> > >
> > > > Provide enough functionality to utilize the mailbox of a memory device.
> > > > The mailbox is used to interact with the firmware running on the memory
> > > > device. The flow is proven with one implemented command, "identify".
> > > > Because the class code has already told the driver this is a memory
> > > > device and the identify command is mandatory.
> > > >
> > > > CXL devices contain an array of capabilities that describe the
> > > > interactions software can have with the device or firmware running on
> > > > the device. A CXL compliant device must implement the device status and
> > > > the mailbox capability. Additionally, a CXL compliant memory device must
> > > > implement the memory device capability. Each of the capabilities can
> > > > [will] provide an offset within the MMIO region for interacting with the
> > > > CXL device.
> > > >
> > > > The capabilities tell the driver how to find and map the register space
> > > > for CXL Memory Devices. The registers are required to utilize the CXL
> > > > spec defined mailbox interface. The spec outlines two mailboxes, primary
> > > > and secondary. The secondary mailbox is earmarked for system firmware,
> > > > and not handled in this driver.
> > > >
> > > > Primary mailboxes are capable of generating an interrupt when submitting
> > > > a background command. That implementation is saved for a later time.
> > > >
> > > > Link: https://www.computeexpresslink.org/download-the-specification
> > > > Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> > > > Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> > >
> > > Hi Ben,
> > >
> > >
> > > > +/**
> > > > + * cxl_mem_mbox_send_cmd() - Send a mailbox command to a memory device.
> > > > + * @cxlm: The CXL memory device to communicate with.
> > > > + * @mbox_cmd: Command to send to the memory device.
> > > > + *
> > > > + * Context: Any context. Expects mbox_lock to be held.
> > > > + * Return: -ETIMEDOUT if timeout occurred waiting for completion. 0 on success.
> > > > + * Caller should check the return code in @mbox_cmd to make sure it
> > > > + * succeeded.
> > >
> > > cxl_xfer_log() doesn't check mbox_cmd->return_code and for my test it currently
> > > enters an infinite loop as a result.
>
> I meant to fix that.
>
> > >
> > > I haven't checked other paths, but to my mind it is not a good idea to require
> > > two levels of error checking - the example here proves how easy it is to forget
> > > one.
>
> Demonstrably, you're correct. I think it would be good to have a kernel only
> mbox command that does the error checking though. Let me type something up and
> see how it looks.

Hi Jonathan. What do you think of this? The bit I'm on the fence about is if I
should validate output size too. I like the simplicity as it is, but it requires
every caller to possibly check output size, which is kind of the same problem
you're originally pointing out.

diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
index 55c5f5a6023f..ad7b2077ab28 100644
--- a/drivers/cxl/mem.c
+++ b/drivers/cxl/mem.c
@@ -284,7 +284,7 @@ static void cxl_mem_mbox_timeout(struct cxl_mem *cxlm,
}

/**
- * cxl_mem_mbox_send_cmd() - Send a mailbox command to a memory device.
+ * __cxl_mem_mbox_send_cmd() - Execute a mailbox command
* @cxlm: The CXL memory device to communicate with.
* @mbox_cmd: Command to send to the memory device.
*
@@ -296,7 +296,8 @@ static void cxl_mem_mbox_timeout(struct cxl_mem *cxlm,
* This is a generic form of the CXL mailbox send command, thus the only I/O
* operations used are cxl_read_mbox_reg(). Memory devices, and perhaps other
* types of CXL devices may have further information available upon error
- * conditions.
+ * conditions. Driver facilities wishing to send mailbox commands should use the
+ * wrapper command.
*
* The CXL spec allows for up to two mailboxes. The intention is for the primary
* mailbox to be OS controlled and the secondary mailbox to be used by system
@@ -304,8 +305,8 @@ static void cxl_mem_mbox_timeout(struct cxl_mem *cxlm,
* not need to coordinate with each other. The driver only uses the primary
* mailbox.
*/
-static int cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
- struct mbox_cmd *mbox_cmd)
+static int __cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm,
+ struct mbox_cmd *mbox_cmd)
{
void __iomem *payload = cxlm->mbox_regs + CXLDEV_MBOX_PAYLOAD_OFFSET;
u64 cmd_reg, status_reg;
@@ -469,6 +470,54 @@ static void cxl_mem_mbox_put(struct cxl_mem *cxlm)
mutex_unlock(&cxlm->mbox_mutex);
}

+/**
+ * cxl_mem_mbox_send_cmd() - Send a mailbox command to a memory device.
+ * @cxlm: The CXL memory device to communicate with.
+ * @opcode: Opcode for the mailbox command.
+ * @in: The input payload for the mailbox command.
+ * @in_size: The length of the input payload
+ * @out: Caller allocated buffer for the output.
+ *
+ * Context: Any context. Will acquire and release mbox_mutex.
+ * Return:
+ * * %>=0 - Number of bytes returned in @out.
+ * * %-EBUSY - Couldn't acquire exclusive mailbox access.
+ * * %-EFAULT - Hardware error occurred.
+ * * %-ENXIO - Command completed, but device reported an error.
+ *
+ * Mailbox commands may execute successfully yet the device itself reported an
+ * error. While this distinction can be useful for commands from userspace, the
+ * kernel will often only care when both are successful.
+ *
+ * See __cxl_mem_mbox_send_cmd()
+ */
+static int cxl_mem_mbox_send_cmd(struct cxl_mem *cxlm, u16 opcode, u8 *in,
+ size_t in_size, u8 *out)
+{
+ struct mbox_cmd mbox_cmd = {
+ .opcode = opcode,
+ .payload_in = in,
+ .size_in = in_size,
+ .payload_out = out,
+ };
+ int rc;
+
+ rc = cxl_mem_mbox_get(cxlm);
+ if (rc)
+ return rc;
+
+ rc = __cxl_mem_mbox_send_cmd(cxlm, &mbox_cmd);
+ cxl_mem_mbox_put(cxlm);
+ if (rc)
+ return rc;
+
+ /* TODO: Map return code to proper kernel style errno */
+ if (mbox_cmd.return_code != CXL_MBOX_SUCCESS)
+ return -ENXIO;
+
+ return mbox_cmd.size_out;
+}
+
/**
* handle_mailbox_cmd_from_user() - Dispatch a mailbox command.
* @cxlmd: The CXL memory device to communicate with.
@@ -1380,33 +1429,18 @@ static int cxl_mem_identify(struct cxl_mem *cxlm)
u8 poison_caps;
u8 qos_telemetry_caps;
} __packed id;
- struct mbox_cmd mbox_cmd = {
- .opcode = CXL_MBOX_OP_IDENTIFY,
- .payload_out = &id,
- .size_in = 0,
- };
int rc;

- /* Retrieve initial device memory map */
- rc = cxl_mem_mbox_get(cxlm);
- if (rc)
- return rc;
-
- rc = cxl_mem_mbox_send_cmd(cxlm, &mbox_cmd);
- cxl_mem_mbox_put(cxlm);
- if (rc)
+ rc = cxl_mem_mbox_send_cmd(cxlm, CXL_MBOX_OP_IDENTIFY, NULL, 0,
+ (u8 *)&id);
+ if (rc < 0)
return rc;

- /* TODO: Handle retry or reset responses from firmware. */
- if (mbox_cmd.return_code != CXL_MBOX_SUCCESS) {
- dev_err(&cxlm->pdev->dev, "Mailbox command failed (%d)\n",
- mbox_cmd.return_code);
+ if (rc < sizeof(id)) {
+ dev_err(&cxlm->pdev->dev, "Short identify data\n",
return -ENXIO;
}

- if (mbox_cmd.size_out != sizeof(id))
- return -ENXIO;
-
/*
* TODO: enumerate DPA map, as 'ram' and 'pmem' do not alias.
* For now, only the capacity is exported in sysfs

[snip]