On Thu, 2021-07-29 at 20:50 +0000, Zev Weiss wrote:

On Thu, Jul 29, 2021 at 01:55:19PM CDT, Winiarska, Iwona wrote:

quoted

On Tue, 2021-07-27 at 17:49 +0000, Zev Weiss wrote:

quoted

On Mon, Jul 12, 2021 at 05:04:41PM CDT, Iwona Winiarska wrote:

quoted

+
+static int peci_detect(struct peci_controller *controller, u8 addr)
+{
+       struct peci_request *req;
+       int ret;
+
+       req = peci_request_alloc(NULL, 0, 0);
+       if (!req)
+               return -ENOMEM;
+

Might be worth a brief comment here noting that an empty request happens
to be the format of a PECI ping command (and/or change the name of the
function to peci_ping()).

I'll add a comment:
"We are using PECI Ping command to detect presence of PECI devices."

Well, what I was more aiming to get at was that to someone not
intimately familiar with the PECI protocol it's not immediately obvious
from the code that it in fact implements a ping (there's no 'msg->cmd =
PECI_CMD_PING' or anything), so I was hoping for something that would
just make that slightly more explicit.

/*
 * PECI Ping is a command encoded by tx_len = 0, rx_len = 0.
 * We expect correct Write FCS if the device at the target address is
 * able to respond.
 */

I would like to avoid doing a peci_ping wrapper that doesn't operate on
peci_device - note that at this point we don't have a struct peci_device yet,
we're using ping to figure out whether we should create one.

quoted

quoted

quoted

+
+/**
+ * peci_request_alloc() - allocate &struct peci_request with buffers
with
given lengths
+ * @device: PECI device to which request is going to be sent
+ * @tx_len: requested TX buffer length
+ * @rx_len: requested RX buffer length
+ *
+ * Return: A pointer to a newly allocated &struct peci_request on
success
or NULL otherwise.
+ */
+struct peci_request *peci_request_alloc(struct peci_device *device, u8
tx_len, u8 rx_len)
+{
+       struct peci_request *req;
+       u8 *tx_buf, *rx_buf;
+
+       req = kzalloc(sizeof(*req), GFP_KERNEL);
+       if (!req)
+               return NULL;
+
+       req->device = device;
+
+       /*
+        * PECI controllers that we are using now don't support DMA,
this
+        * should be converted to DMA API once support for controllers
that
do
+        * allow it is added to avoid an extra copy.
+        */
+       if (tx_len) {
+               tx_buf = kzalloc(tx_len, GFP_KERNEL);
+               if (!tx_buf)
+                       goto err_free_req;
+
+               req->tx.buf = tx_buf;
+               req->tx.len = tx_len;
+       }
+
+       if (rx_len) {
+               rx_buf = kzalloc(rx_len, GFP_KERNEL);
+               if (!rx_buf)
+                       goto err_free_tx;
+
+               req->rx.buf = rx_buf;
+               req->rx.len = rx_len;
+       }
+

As long as we're punting on DMA support, could we do the whole thing in
a single allocation instead of three?  It'd add some pointer arithmetic,
but would also simplify the error-handling/deallocation paths a bit.

Or, given that the one controller we're currently supporting has a
hardware limit of 32 bytes per transfer anyway, maybe just inline
fixed-size rx/tx buffers into struct peci_request and have callers keep
them on the stack instead of kmalloc()-ing them?

I disagree on error handling (it's not complicated) - however, one argument
for
doing a single alloc (or moving the buffers as fixed-size arrays inside
struct
peci_request) is that single kzalloc is going to be faster than 3. But I
don't
expect it to show up on any perf profiles for now (since peci-wire interface
is
not a speed demon).

I wanted to avoid defining max size for TX and RX in peci-core.
Do you have a strong opinion against multiple alloc? If yes, I can go with
fixed-size arrays inside struct peci_request.

As is it's certainly not terribly complicated in an absolute sense, but
comparatively speaking the cleanup path for a single allocation is still
simpler, no?

Making it more efficient would definitely be a nice benefit too (perhaps
a more significant one) -- in a typical deployment I'd guess this code
path will see roughly socket_count + total_core_count executions per
second?  On a big multi-socket system that could end up being a
reasonably large number (>100), so while it may not end up as a major
hot spot in a system-wide profile, it seems like it might be worth
having it do 1/3 as many allocations if it's reasonably easy to do.
(And while I don't think the kernel is generally at fault for this, from
what I've seen of OpenBMC as a whole I think it might benefit from a bit
more overall frugality with CPU cycles.)

As for a fixed max request size and inlined buffers, I definitely
understand not wanting to put a cap on that in the generic PECI core --
and actually, looking at the peci-npcm code from previous iterations of
the PECI patchset, it looks like the Nuvoton hardware has significantly
larger size limits (127 bytes if I'm reading things right) that might be
a bit bulky for on-stack allocation.  So while that's appealing
efficiency-wise and (IMO) aesthetically, perhaps it's not ultimately
real viable.

Hmm, though (thinking out loud) I suppose we could also get down to a
zero-allocation common case by having the driver hold on to a request
struct and reuse it across transfers, given that they're all serialized
by a mutex anyway?

With the "zero-allocation" case we still need some memory to copy the necessary
data from the "request area" (now "global" - per-controller).

After more consideration, I think this doesn't have to rely on controller
capabilities, we can just define a max value based on the commands we're using
and use that with single alloc (with rx and tx having fixed size arrays).
I'll change it in v2.

Thank you
-Iwona