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Message-ID: <30fd2355-0db0-4d9c-bcfe-88b06b8aaeae@icloud.com>
Date: Wed, 28 Aug 2024 18:48:28 +0800
From: Zijun Hu <zijun_hu@...oud.com>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>,
Ira Weiny <ira.weiny@...el.com>, Dave Jiang <dave.jiang@...el.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Rafael J. Wysocki" <rafael@...nel.org>, Davidlohr Bueso
<dave@...olabs.net>, Dave Jiang <dave.jiang@...el.com>,
Alison Schofield <alison.schofield@...el.com>,
Vishal Verma <vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>,
Dan Williams <dan.j.williams@...el.com>,
Takashi Sakamoto <o-takashi@...amocchi.jp>, Timur Tabi <timur@...nel.org>,
"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org,
netdev@...r.kernel.org, Zijun Hu <quic_zijuhu@...cinc.com>
Subject: Re: [PATCH v3 2/3] cxl/region: Find free cxl decoder by
device_for_each_child()
On 2024/8/27 19:53, Zijun Hu wrote:
> On 2024/8/27 19:30, Jonathan Cameron wrote:
>> On Sat, 24 Aug 2024 17:07:44 +0800
>> Zijun Hu <zijun_hu@...oud.com> wrote:
>>
>>> From: Zijun Hu <quic_zijuhu@...cinc.com>
>>>
>>> To prepare for constifying the following old driver core API:
>>>
>>> struct device *device_find_child(struct device *dev, void *data,
>>> int (*match)(struct device *dev, void *data));
>>> to new:
>>> struct device *device_find_child(struct device *dev, const void *data,
>>> int (*match)(struct device *dev, const void *data));
>>>
>>> The new API does not allow its match function (*match)() to modify
>>> caller's match data @*data, but match_free_decoder() as the old API's
>>> match function indeed modifies relevant match data, so it is not suitable
>>> for the new API any more, solved by using device_for_each_child() to
>>> implement relevant finding free cxl decoder function.
>>>
>>> Suggested-by: Ira Weiny <ira.weiny@...el.com>
>>> Signed-off-by: Zijun Hu <quic_zijuhu@...cinc.com>
>> This seems to functionally do the same as before.
>>
>
> yes, this change have the same logic as previous existing logic.
>
>> I'm not sure I like the original code though so a comment inline.
>>
>>> ---
>>> drivers/cxl/core/region.c | 30 ++++++++++++++++++++++++------
>>> 1 file changed, 24 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>>> index 21ad5f242875..c2068e90bf2f 100644
>>> --- a/drivers/cxl/core/region.c
>>> +++ b/drivers/cxl/core/region.c
>>> @@ -794,10 +794,15 @@ static size_t show_targetN(struct cxl_region *cxlr, char *buf, int pos)
>>> return rc;
>>> }
>>>
>>> +struct cxld_match_data {
>>> + int id;
>>> + struct device *target_device;
>>> +};
>>> +
>>> static int match_free_decoder(struct device *dev, void *data)
>>> {
>>> + struct cxld_match_data *match_data = data;
>>> struct cxl_decoder *cxld;
>>> - int *id = data;
>>>
>>> if (!is_switch_decoder(dev))
>>> return 0;
>>> @@ -805,17 +810,31 @@ static int match_free_decoder(struct device *dev, void *data)
>>> cxld = to_cxl_decoder(dev);
>>>
>>> /* enforce ordered allocation */
>>> - if (cxld->id != *id)
>>> + if (cxld->id != match_data->id)
>>
>> Why do we carry on in this case?
>> Conditions are:
>> 1. Start match_data->id == 0
>> 2. First pass cxld->id == 0 (all good) or
>> cxld->id == 1 say (and we skip until we match
>> on cxld->id == 0 (perhaps on the second child if they are
>> ordered (1, 0, 2) etc.
>>
>> If we skipped and then matched on second child but it was
>> already in use (so region set), we will increment match_data->id to 1
>> but never find that as it was the one we skipped.
>>
>> So this can only work if the children are ordered.
>> So if that's the case and the line above is just a sanity check
>> on that, it should be noisier (so an error print) and might
>> as well fail as if it doesn't match all bets are off.
>>
>
> it seems Ira Weiny also has some concerns related to previous existing
> logic as following:
>
> https://lore.kernel.org/all/66c4a136d9764_2ddc2429435@iweiny-mobl.notmuch/
> "Also for those working on CXL I'm questioning the use of ID here and
> the dependence on the id's being added to the parent in order. Is that
> a guarantee?"
>
Hi Jonathan, ira
i don't know CXL decoder at all. For your concerns about original logic:
is it okay to find a child with the minimal index as shown below ?
i would like to submit it as a separate patch if you like it.
diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 21ad5f242875..d10cca02eba8 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -797,21 +797,27 @@ static size_t show_targetN(struct cxl_region
*cxlr, char *buf, int pos)
static int match_free_decoder(struct device *dev, void *data)
{
struct cxl_decoder *cxld;
- int *id = data;
+ struct cxl_decoder *target_cxld;
+ struct device **target_device = data;
if (!is_switch_decoder(dev))
return 0;
cxld = to_cxl_decoder(dev);
- /* enforce ordered allocation */
- if (cxld->id != *id)
+ if (cxld->region)
return 0;
- if (!cxld->region)
- return 1;
+ if (!*target_device) {
+ *target_device = get_device(dev);
+ return 0;
+ }
- (*id)++;
+ target_cxld = to_cxl_decoder(*target_device);
+ if (cxld->id < target_cxld->id) {
+ put_device(*target_device);
+ *target_device = get_device(dev);
+ }
return 0;
}
@@ -839,8 +845,7 @@ cxl_region_find_decoder(struct cxl_port *port,
struct cxl_endpoint_decoder *cxled,
struct cxl_region *cxlr)
{
- struct device *dev;
- int id = 0;
+ struct device *dev = NULL;
if (port == cxled_to_port(cxled))
return &cxled->cxld;
@@ -849,7 +854,7 @@ cxl_region_find_decoder(struct cxl_port *port,
dev = device_find_child(&port->dev, &cxlr->params,
match_auto_decoder);
else
- dev = device_find_child(&port->dev, &id,
match_free_decoder);
+ device_for_each_child(&port->dev, &dev, match_free_decoder);
if (!dev)
return NULL;
/*
> perhaps, create a new dedicated thread to discuss original design.
>
>> Jonathan
>>
>>> return 0;
>>>
>>> - if (!cxld->region)
>>> + if (!cxld->region) {
>>> + match_data->target_device = get_device(dev);
>>> return 1;
>>> + }
>>>
>>> - (*id)++;
>>> + match_data->id++;
>>>
>>> return 0;
>>> }
>>>
>>> +/* NOTE: need to drop the reference with put_device() after use. */
>>> +static struct device *find_free_decoder(struct device *parent)
>>> +{
>>> + struct cxld_match_data match_data = {
>>> + .id = 0,
>>> + .target_device = NULL,
>>> + };
>>> +
>>> + device_for_each_child(parent, &match_data, match_free_decoder);
>>> + return match_data.target_device;
>>> +}
>>> +
>>> static int match_auto_decoder(struct device *dev, void *data)
>>> {
>>> struct cxl_region_params *p = data;
>>> @@ -840,7 +859,6 @@ cxl_region_find_decoder(struct cxl_port *port,
>>> struct cxl_region *cxlr)
>>> {
>>> struct device *dev;
>>> - int id = 0;
>>>
>>> if (port == cxled_to_port(cxled))
>>> return &cxled->cxld;
>>> @@ -849,7 +867,7 @@ cxl_region_find_decoder(struct cxl_port *port,
>>> dev = device_find_child(&port->dev, &cxlr->params,
>>> match_auto_decoder);
>>> else
>>> - dev = device_find_child(&port->dev, &id, match_free_decoder);
>>> + dev = find_free_decoder(&port->dev);
>>> if (!dev)
>>> return NULL;
>>> /*
>>>
>>
>
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