Re: [PATCH v2 2/4] cxl/region: Prevent device_find_child() from modifying caller's match data

Zijun Hu
2024-08-21T12:46:25Z

On 2024/8/20 21:59, Ira Weiny wrote:
> Zijun Hu wrote:
>> From: Zijun Hu <quic_zijuhu@quicinc.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, fixed by implementing a equivalent
>> cxl_device_find_child() instead of the old API usage.
>
> Generally it seems ok but I think some name changes will make this more
> clear. See below.
>

okay.

> 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?
>
>>
>> Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
>> ---
>> drivers/cxl/core/region.c | 36 +++++++++++++++++++++++++++++++++++-
>> 1 file changed, 35 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>> index 21ad5f242875..8d8f0637f7ac 100644
>> --- a/drivers/cxl/core/region.c
>> +++ b/drivers/cxl/core/region.c
>> @@ -134,6 +134,39 @@ static const struct attribute_group *get_cxl_region_access1_group(void)
>> return &cxl_region_access1_coordinate_group;
>> }
>>
>> +struct cxl_dfc_data {
>
> struct cxld_match_data
>
> 'cxld' == cxl decoder in our world.
>

make sense.

>> + int (*match)(struct device *dev, void *data);
>> + void *data;
>> + struct device *target_device;
>> +};
>> +
>> +static int cxl_dfc_match_modify(struct device *dev, void *data)
>
> Why not just put this logic into match_free_decoder?
>

Actually, i ever considered solution B as you suggested in the end.

For this change, namely, solution A:
1) this change is clearer and easier to understand.
2) this change does not touch any existing cxld logic

For solution B:
it is more reasonable

i finally select A since it can express my concern and relevant solution
clearly.

>> +{
>> + struct cxl_dfc_data *dfc_data = data;
>> + int res;
>> +
>> + res = dfc_data->match(dev, dfc_data->data);
>> + if (res && get_device(dev)) {
>> + dfc_data->target_device = dev;
>> + return res;
>> + }
>> +
>> + return 0;
>> +}
>> +
>> +/*
>> + * I have the same function as device_find_child() but allow to modify
>> + * caller's match data @*data.
>> + */
>
> No need for this comment after the new API is established.
>

i have given up the idea within v1 to introduce a new API which *should
ONLY* be used by this patch series, so it is not worthy of a new API
even if it can bring convenient for this patch series.

>> +static struct device *cxl_device_find_child(struct device *parent, void *data,
>> + int (*match)(struct device *dev, void *data))
>> +{
>> + struct cxl_dfc_data dfc_data = {match, data, NULL};
>> +
>> + device_for_each_child(parent, &dfc_data, cxl_dfc_match_modify);
>> + return dfc_data.target_device;
>> +}
>> +
>> static ssize_t uuid_show(struct device *dev, struct device_attribute *attr,
>> char *buf)
>> {
>> @@ -849,7 +882,8 @@ 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 = cxl_device_find_child(&port->dev, &id,
>> + match_free_decoder);
>
> This is too literal. How about the following (passes basic cxl-tests).
>

it is reasonable.

do you need me to submit that you suggest in the end and add you as
co-developer ?

OR

you submit it by yourself ?

either is okay for me.

> Ira
>
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 21ad5f242875..c1e46254efb8 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 +810,30 @@ 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)
> return 0;
>
> - if (!cxld->region)
> + if (!cxld->region && get_device(dev)) {

get_device(dev) failure may cause different logic against existing
but i think it should be impossible to happen normally.

> + match_data->target_device = dev;
> return 1;
> + }
>
> - (*id)++;
> + match_data->id++;
>
> return 0;
> }
>
> +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 +858,6 @@ 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 +866,8 @@ 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;
> /*